Novak Djokovic of Serbia hits a return to Roger Federer of Switzerland during the men’s final on the fourteenth day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 13 September 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/CJ GUNTHER

Novak Djokovic of Serbia in action against Roger Federer of Switzerland during their final match at the BNP Paribas Open tennis tournament in Indian Wells, California, USA, 22 March 2015. Djokovic won the final in three sets. EPA/DANIEL MURPHY

Novak Djokovic of Serbia hits a return to Roberto Bautista Agut of Spain during their match on the seventh day of the 2015 US Open Tennis Championship at the USTA National Tennis Center in Flushing Meadows, New York, USA, 06 September 2015. The US Open runs through 13 September, which is a return to a 14-day schedule. EPA/JASON SZENES

Novak Djokovic of Serbia returns to Bernard Tomic of Australia in their third round match during the Wimbledon Championships at the All England Lawn Tennis Club, in London, Britain, 03 July 2015. EPA/SEAN DEMPSEY

Novak Djokovic of Serbia in action against Thanasi Kokkinakis of Australia during their third round match for the French Open tennis tournament at Roland Garros in Paris, France, 30 May 2015. EPA/ROBERT GHEMENT