Milos Raonic of Canada serves against Roger Federer of Switzerland during their round robin match during the Barclays ATP World Tour Finals at the O2 Arena on November 9, 2014 in London, England.

Roger Federer of Switzerland serves against Milos Raonic of Canada during their round robin match during the Barclays ATP World Tour Finals at the O2 Arena on November 9, 2014 in London, England.

Roger Federer of Switzerland stretches to play a forehand against Milos Raonic of Canada during their round robin match during the Barclays ATP World Tour Finals at the O2 Arena on November 9, 2014 in London, England.

Roger Federer of Switzerland in action in his match against Milos Raonic of Canada in the round robin during day one of the Barclays ATP World Tour Finals tennis at O2 Arena on November 9, 2014 in London, England.

Milos Raonic of Canada in action in his match against Roger Federer of Switzerland in the round robin during day one of the Barclays ATP World Tour Finals tennis at O2 Arena on November 9, 2014 in London, England.

Roger Federer of Switzerland in action in his match against Milos Raonic of Canada in the round robin during day one of the Barclays ATP World Tour Finals tennis at O2 Arena on November 9, 2014 in London, England.

Roger Federer of Switzerland celebrates winning a game in his match against Milos Raonic of Canada in the round robin during day one of the Barclays ATP World Tour Finals tennis at O2 Arena on November 9, 2014 in London, England.

Roger Federer of Switzerland celebrates winning a game in his match against Milos Raonic of Canada in the round robin during day one of the Barclays ATP World Tour Finals tennis at O2 Arena on November 9, 2014 in London, England.

Milos Raonic of Canada shows his emotions against Roger Federer of Switzerland during their round robin match during the Barclays ATP World Tour Finals at the O2 Arena on November 9, 2014 in London, England.