Roger Federer of Switzerland hits a return to Stan Wawrinka of Switzerland during their Semifinals round match on the twelfth day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 11 September 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/JASON SZENES

Roger Federer of Switzerland in action during his fourth round match against Jack Sock of the US at the BNP Paribas Open tennis tournament in Indian Wells, California, USA, 18 March 2015. EPA/DANIEL MURPHY

Roger Federer of Switzerland in action against his compatriot Stan Wawrinka during their quarterfinal match for the French Open tennis tournament at Roland Garros in Paris, France, 02 June 2015. EPA/IAN LANGSDON

Roger Federer of Switzerland returns to Sam Querry of USA in their second round match during the Wimbledon Championships at the All England Lawn Tennis Club, in London, Britain, 02 July 2015. EPA/FACUNDO ARRIZABALAGA

Roger Federer of Switzerland in action against Pablo Cuevas of Uruguay during their match in the Western & Southern Open at the Linder Family Tennis Center in Mason, Ohio, USA, 18 August 2015. EPA/TANNEN MAURY