Roger Federer has admitted that he will find it tough to defeat Andy Murray in the semi-finals of the ATP World Tour Finals at London's O2 Arena on Sunday.
The Scot has won the last two meetings against the 31-year-old, who believes that his opponent is in his category.
"To me, he was always going to be a player who needed just a bit more time than let's say a [Rafael] Nadal or a [Novak] Djokovic," BBC Sport quotes Federer as saying.
"He's more in my category where he just needed more time to fill out [his] game. But when it's all really happening, I think it's a great package for Andy.
"Right now, I think he's playing great tennis. Obviously the last six months have been amazing for him. I expect him to be extremely tough in the semis."
In the other semi-final on Sunday, Djokovic will play Juan Martin del Potro.
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