Roger Federer of Switzerland returns the ball against Milos Raonic of Canada in the singles group B match at the men’s ATP World Tour Finals tennis tournament at the O2 Arena in London, Britain, 09 November 2014. EPA/FACUNDO ARRIZABALAGA

Switzerland’s Roger Federer following his two sets win over Japan’s Kei Nishikori at the ATP World Tour Finals mens tennis tournament at the O2 Arena in London, Britain, 11 November 2014. EPA/ANDY RAIN

Switzerland’s Roger Federer returns to Japan’s Kei Nishikori at the ATP World Tour Finals mens tennis tournament at the O2 Arena in London, Britain, 11 November 2014. EPA/ANDY RAIN

Switzerland’s Roger Federer returns to Britain’s Andy Murray during a round robin match at the ATP World Tour Finals mens tennis tournament at the O2 Arena in London, Britain, 13 November 2014. EPA/ANDY RAIN

Switzerland’s Roger Federer reacts during his match with Switzerland’s Stanislas Wawrinka during the ATP World Tour Finals semi final match at the O2 Arena in London, Britain, 15 November 2014. EPA/ANDY RAIN

Switzerland’s Roger Federer pulls out of the ATP World Tour Finals final with injury ceding the win to Serbia’s Novak Djokovic during the ATP World Tour Finals at the O2 Arena in London, Britain, 16 November 2014. EPA/ANDY RAIN