Live Commentary: WTA Championships - Serena Williams vs. Jelena Jankovic - as it happened

Good afternoon everyone. Li Na has already booked her spot in tomorrow's final but who will join here? Serena Williams is the favourite but she will have to overcome Jelena Jankovic. I'm here to take you through the action today.

Williams has been the best performer in this tournament this week and the majority of people have tipped her to lift the title tomorrow but a shock is always possible in this sport. Jankovic will take confidence from the fact that she has beaten the American four times in her career. She must produce her best form today.

Williams is aiming to defend her title this week but she won't be looking too far ahead. She looks as focused as ever during he pre-match preparations.

Can Jankovic produce a shock today? She only won once in the group stages but her superior sets records saw her beat rivals to this semi-final spot. It's imperative that she starts this match well.

Both of these players will be desperate for a good start here and the first few games could prove to be crucial. The final preparations have been made and it will be Jankovic who serves first. Play!

Great start from Williams, who hammers a forehand past Jankovic as the Serb rushes to the net. Williams attempts to dominate the next two rallies but a couple of unforced errors hands the lead to Jankovic. A good second serve from Jankovic gives her a 40-15 lead and the hold is secured when Williams strikes the net with a volley. Solid start.

Jankovic moves well to connect when a serve from Williams is pushed out wide but the American opens her body to fire a winner down the line. Two big serves prevent Jankovic from starting a rally and a second ace allows Williams to quickly secure a hold.

Great work from Jankovic, who stays in the point long enough to force Williams into a difficult smash following a well placed lob. Williams approaches the net during the next point but her forehand is too loose and the ball drifts wide. Another venture forward is far more successful as the top seed gives Jankovic no chance with a powerful smash. Superb tennis on show early on, with Jankovic moving her opponent around the court before sending her the wrong way with a classy backhand. Williams fires a return straight into the net and Jankovic wins the game.

BREAK! Excellent from Jankovic as she continues her impressive start with a great return which brings an unforced error from Williams. The defending champion is invited towards the net to volley a winner when Jankovic stretches to return a backhand. A big serve can't be returned, despite the Serb's best efforts. However, Jankovic's confidence is then boosted by a brilliant smash on the back foot. It's break point Jankovic when Williams finds the net attempting to return a powerful forehand. The American responds well by following a wide serve with a great forehand winner. Jankovic attacks a second serve but the ball crashes into the net. Williams fails with her first opportunity on advantage and Jankovic responds to find a second break point when she meets a drop volley to guide the ball past her rival. Williams hits a forehand wide and the break is secured. Massive moment!

BREAK! We haven't seen the usual intensity from Williams yet but she moves into a 0-30 lead thanks to a couple of powerful backhands. Jankovic pulls a point back when she retreats to smash another winner. A mistake from the Serbian player gifts Williams two break points, the first of which is wasted when a forehand finds the net. However, Jankovic then sends a backhand long and Williams has immediately broken back.

Williams had her head buried in her towel during the break. She looks to be struggling with something but the problem isn't clear. She definitely looks a bit flat out there but that doesn't stop her cruising through the game thanks to some big serves and a couple of powerful ground strokes.

BREAK! Classic Williams as he reaches a wide serve to fire a return winner back across the court. Jankovic looks slightly shaken as she falls further behind due to an unforced error. Williams easily reaches a drop shot to earn herself three break points. Jankovic plays a desperate backhand and the loose shot gifts Williams a crucial break of serve.

There has been some reports that Williams was crying a few moments during the change of ends. She makes it 10 points in a row since that bizarre moment though to move into the lead at the start of a new service game. An unforced error gifts Jankovic a point but a brutal forehand is too hot to handle for the seventh seed. A big serve then allows Williams to secure the hold.

We definitely aren't seeing the Williams we usually do but her class has helped her through this match so far. Jankovic settles into a good rhythm with her serves and some mistakes from Williams make it an easy win to 15 for the Serb.

SET! Reports on Twitter are now suggesting that Williams could be struggling with a stomach injury. Jankovic does well to connect with some big serves early on but Williams uses the court well to win the first two points. The Serb executes another smash shot to perfection before an unforced error makes the score 30-30. Brilliant from Jankovic as she displays great pace to reach a drop shot, with a surprised Williams firing the next shot into the net. Is the reported injury getting worse? Williams doesn't look too happy, despite following a forehand winner with an ace down the middle. Jankovic piles on the pressure by forcing deuce with a classy return winner on the backhand side. Williams wastes a break point when a lazy backhand hits the net halfway up. However, Jankovic gifts Williams a third set point when she attempts to hit a winner on her return. Another unforced error finally hands the set to Williams. That was a big battle for the top seed, who still appears to be struggling with something.

I don't know if it's a problem off the court or an injury but there's certainly something wrong with Williams, with yet more reports suggesting that she was crying before the game and in the middle of that first set. Jankovic had her moments but she needs to take advantage when she gets opportunities in the next set.

Confident start from Jankovic following the first set loss. A clearly subdued Williams fails with a couple of big forehands early on and Jankovic then shows great determination to stay in a long rally before seeing another forehand from her opponent go long. A big serve is not returned and the first game of the second set is won by Jankovic.

Williams isn't moving brilliantly around the court but her power causes problems for her opponent early in the game as he eases into a 30-0 lead. Jankovic delights the crowd when she sprints to meet a drop shot and surprises Williams by drilling a cross-court backhand winner. A stunning half-volley earns Williams a point at a crucial stage in the game but Jankovic fights back again when a lob from the American goes long. However, a miscued forehand from the seventh seed then hands the game to Williams.

Jankovic fails with a challenge on her first serve but the first point goes to the Serb when Williams fires a backhand long. A good is serve is followed by some clever ground strokes and an impatient Williams misses the baseline again. Jankovic then produces her first ace of the match before handing a point back when she finds the net. Another error from Williams allows Jankovic to complete the hold of serve.

SET! Jankovic has been returning the ball with positive intent so far and that attitude earns her two points early in the game. Williams continues to struggle with her movement on the baseline and a lazy slice fails to beat the net, offering three break points to her opponent. The American saves the first one but the next forehand goes long and Jankovic breaks. Big moment in the set!

Williams barely breaks into a jog as Jankovic uses the court to her advantage to work the ball into good areas and secure a 30-0 love lead. She then finds three game points when she meets her opponent's lob with a perfect smash. Williams pulls a point back when Jankovic fires a forehand slightly wide. However, a stunning ace secures the hold of serve.

BREAK! Williams hands the control to Jankovic with a weak second serve and the Serb takes advantage by following a powerful forehand with another great overhead smash. The top seed responds magnificently as she sends her rival out wide before hitting a forehand winner into an open court. An unforced error on the forehand gives Jankovic a 15-30 lead and two break points arrive when Williams gives up on a point, prompting jeers from a disappointed crowd. The opportunity is taken by Jankovic, who secures another break of serve when Williams finds the net with a forehand.

BREAK!We could continue to speculate about a possible injury but if the problem for Williams is physical, why has the trainer not been called? An unforced error hands the first point of the game to Williams and the American continues to fight as she earns a 15-40 lead due to some impressive ground strokes. Jankovic pulls a point back when Williams struggles to return a big serve. The break of serve is then confirmed when Williams executes a superb backhand winner down the line. That's one break snatched back.

SET! Williams and Jankovic exchange unforced errors early in a new game. The American fights the urge to approach the net in a longer rally before unleashing a stunning cross-court backhand winner. Jankovic responds brilliantly with a return winner on the backhand side. A set point arrives when Jankovic fires another smash past her opponent. The match is level when Williams hits the net with her forehand. We're going into a third set!

Williams is providing some stunning moments followed by periods of disappointing tennis. She hasn't looked right since arriving on court but credit goes to Jankovic, who took full advantage in the second set. A shock looks possible in Istanbul.

BREAK! Great return from Williams at the start of the third set to take the lead. Jankovic's forehand is slightly weak, which invites a powerful cross-court backhand winner from her opponent. Williams spoils a promising start to the game by making three straight mistakes. A double fault from Jankovic brings deuce and a forehand into the net hands the advantage to Williams. The American spots an opening when Jankovic approaches the net but her forehand hammers into the top of the net. Jankovic does well to compete in the next rally but Williams displays her class by clipping the line with a forehand winner. Jankovic hesitates when she thinks Williams has hit her shot long but a challenge is unsuccessful. Big break for Williams!

BREAK! Williams begins the service game with an ace. Jankovic is fortunate to see a backhand just creep over the net moments later. The Serb handles her opponent's power in a long rally before approaching the net to guide a forehand into a big open space to the left of Williams. A double fault from the top seed gifts Jankovic a couple of break points but the first one is saved when Williams sends her the wrong way with the forehand. Jankovic breaks! A backhand goes long and that's a perfect response by the underdog.

BREAK! Stunning tennis from Williams. She brings an unforced error from her rival with some brutal ground strokes before following that point with a fantastic return winner. Jankovic makes it 15-30 with an ace and the scores are level in the game when Williams misses the line with her backhand. A break point then arrives when Williams drills a forehand into the corner of the court. There's some signs of the Williams we are used to seeing as she shows incredible power to secure the break with a backhand winner.

Jankovic continues to attack the second serve and her confidence earns her the first point of the game when Williams hits a backhand into the net. The Serb is stretched fully by a clever Williams, who moves into a 30-15 lead before moving further ahead with a big serve down the middle. It's a confident hold from Williams as Jankovic's mistake hands the game to her opponent.

BREAK! Jankovic is beginning to drop some of her ground strokes short, which is inviting Williams to dip into her bag of tricks. It's the backhand which provides two winners early in the game for the American. Williams does very well to force Jankovic to play an extra couple of shots during a long rally but the Serb continues to smash impressively. The top seed somehow manages to return a backhand down the line but she's left frustrated when her next shot strikes the net. It's the turn of Williams to smash home a winner when Jankovic attempts a lob. Jankovic plays a wild forehand on break point and Williams can almost reach out to touch a place in the final.

Jankovic fails to return a big serve early in the game but she pulls a point back when Williams lob lands out. An approach to the net ends successfully when Williams gives her rival no chance to return a brutal forehand. The American targets the line with her next forehand but she's a couple of inches off target. Jankovic's commitment is clear for everyone to see as she somehow manages to stay in a very long rally but Williams refuses to take her foot off the gas as he fires another winner home, prompting a rare sign of emotion from her this afternoon. An error from Williams makes it deuce but she secures advantage with her fourth ace of the afternoon. A volley wide from Jankovic makes it 5-1 to a clearly rejuvenated Williams.

Williams is playing her best tennis when it matters most. Jankovic sends a return wide when her opponent unleashes a powerful cross-court backhand. Williams hits her next shot long when she targets a winner off Jankovic's serve and the Serb then moves ahead with a line-clipping backhand. The American seems to be caught in two minds when she drills a smash into the net. Williams then drives a forehand long to gift the game to Jankovic.

BREAK! Williams loses the first point when she sends a lazy forehand into the net. Jankovic receives some more support from the Istanbul crowd when Williams hits a backhand long. A serve is pushed wider by the first seed and Jankovic is unable to return the ball. Williams makes it 30-30 when she produces a gorgeous forehand winner to Jankovic's left. The Serb then fails to connect properly with a forehand and that gifts match point to Williams. It should be all over immediately but Williams finds the net with a weak forehand. Deuce becomes advantage Jankovic when the top seed slices a backhand wide. Wow! This game isn't over yet as an unforced error hands the break to Jankovic.

Jankovic seems to be moving around the court without much effort but she surrenders an early lead in the game when a backhand into the net makes it 15-15. Williams attacks the next first serve but she also finds the net. Loose, wild, call it what you want but a dreadful shot from Williams hands her rival a 40-15 lead. Jankovic follows that with a big serve to secure the hold.

GAME, SET, MATCH! Williams sends down an impressive second serve which proves to be too difficult to return. A wider one is returned by a desperate Jankovic but a big forehand makes it 30-0. The Serb pulls one back when Williams fires a backhand long. However, the top seed has two match points when Jankovic hammers a return into the net. A couple of poor serves allows Jankovic to pull a point back and another backhand is then fired long. It's not over just yet though as Jankovic claims a break point with a well placed winner. Williams makes it deuce again with an ace before taking the advantage with a fantastic forehand winner. She's done it! Williams ends the match with a convincing smash to book her spot in the final.

I don't think many people were expecting for this match to be so evenly contested. Jankovic pushed Williams all the way but the American's quality showed when it mattered most. She will meet Li Na in tomorrow's final.

It remains to be seen whether we'll find out more about what was bothering Williams throughout this match but she managed to fight off those worries to continue her impressive streak in this tournament.

We've seen some great drama this afternoon in Istanbul. Williams proved to everyone that she will always be difficult to beat, even when she is not at her best. Thanks for joining me today. Goodbye.