John Isner of US returns the ball to Roger Federer of Switzerland during their round of sixteen match at the BNP Paribas 2015 Masters tennis tournament in Paris, France, 05 November 2015. EPA/YOAN VALAT

Novak Djokovic of Serbia focusses the ball while returning to Gilles Simon of France during their round of sixteen match at the BNP Paribas 2015 Masters tennis tournament in Paris, France, 05 November 2015. EPA/YOAN VALAT

David Ferrer of Spain serves the ball to Grigor Dimitrov of Bulgaria during their round of sixteen match at the BNP Paribas 2015 Masters tennis tournament in Paris, France, 05 November 2015. EPA/YOAN VALAT

Kei Nishikori of Japan wipes his face with a towel during his round of sixteen match against Richard Gasquet of France at the BNP Paribas 2015 Masters tennis tournament in Paris, France, 05 November 2015. EPA/YOAN VALAT

Gilles Simon of France returns the ball to Novak Djokovic of Serbia during their round of sixteen match at the BNP Paribas 2015 Masters tennis tournament in Paris, France, 05 November 2015. EPA/YOAN VALAT