French number one Jo-Wilfried Tsonga has admitted that it is "always difficult" to play Roger Federer.
Tsonga failed to beat his Swiss opponent during the opening match of ATP World Tour Finals earlier today, the second time Federer has beaten him in as many weeks.
After the match, Tsonga revealed that Federer's speed is one of his main assets.
"I think he was a bit surprised because I played so bad in the first set, then I played correctly in the second, but it's always difficult because he's really quick," Tsonga told BBC Sport.
"Sometimes you think you will get the point but Roger is still there. That's why it's tough to play against him."
The other two players in Group A are Rafael Nadal and Mardy Fish, who face each other later this evening.
