“We played very well in the first half,” said Blues boss Sarri. “But as usual in the last two months we started very badly in the second half.

“We need a solution. In the last 10 matches we started like in this match, it’s a big problem.

“Probably at the end of the first half we thought we had qualified, but against this opponent it was not true. I have never seen something like these opponents physically, so it was clear the second half could be difficult for us.

“But we did something wrong in the first 10 minutes of the second half.

“At half-time I told them to start with the same application as they started the first, but we didn’t do it. It’s not easy to understand.

“I could try without going in the dressing room at half-time if I could solve the problem.”

Pedro opened the scoring following a flowing move involving Cesar Azpilicueta and Olivier Giroud, with the Spaniard applying the finish.

Pedro should have claimed the second but he somehow side-footed Eden Hazard’s inviting ball across goal on to the post.

Fortunately for the Blues the ball ricocheted into luckless defender Simon Deli and bounced into the net.

N’Golo Kante then set Pedro through to tee up Giroud for his 10th goal in the competition this season.

That should have been that, but Chelsea’s defence dozed off in the 26th minute to allow the unmarked Tomas Soucek to pull one back with a header at the far post.

Chelsea merely went straight down the other end and restored their four-goal cushion, Pedro pouncing on the rebound after Giroud’s close-range effort was saved.

Yet a 5-1 aggregate lead at half-time became 5-3 only 10 minutes into the second half, a quickfire double from Petr Sevcík raising the Czech side’s hopes of a staging remarkable comeback.

But Chelsea weathered the storm to set up a semi-final against Eintracht Frankfurt.

Hazard, the subject of a couple of clattering early challenges, was taken off midway through the second half, but Sarri hopes he will be fit to face Burnley on Monday.

“It’s nothing serious, I hope,” he added. “It’s only a strong knock. But I think we can recover him for the next match on Monday.”

Disappointed Slavia coach Jindrich Trpisovsky said: “We scored three goals, but it is only a little remedy after the first half.”

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