Andy Murray believes that Roger Federer will prove to be a difficult opponent in the semi-finals of the ATP World Tour Finals at the O2 Arena in London on Sunday.
The US Open champion says that the Swiss is "great" in indoors competition and admits that he has struggled against him indoors in recent times.
"He's played great indoors the last few years, that's for sure," BBC Sport quotes Murray as saying. "I've played him quite a few times indoors and had decent success against him early on.
"The last few times I've played him, I haven't done so well indoors, but his record speaks for itself.
"He likes the conditions when they're like this, it helps his timing. When he's timing the ball well, he plays great tennis."
In the other semi-final on Sunday, Novak Djokovic will play Juan Martin del Potro.
