Novak Djokovic of Serbia in action against Roger Federer of Switzerland during the final of the Western & Southern Open at the Linder Family Tennis Center in Mason, near Cincinnati, Ohio, USA, 23 August 2015. EPA/TANNEN MAURY

Roger Federer of Switzerland in action against Novak Djokovic of Serbia during the final of the Western & Southern Open at the Linder Family Tennis Center in Mason, near Cincinnati, Ohio, USA, 23 August 2015. EPA/TANNEN MAURY

Novak Djokovic of Serbia in action against Roger Federer of Switzerland during the final of the Western & Southern Open at the Linder Family Tennis Center in Mason, near Cincinnati, Ohio, USA, 23 August 2015. EPA/TANNEN MAURY

Roger Federer of Switzerland leaps to try to make a return shot to Novak Djokovic of Serbia during the final of the Western & Southern Open at the Linder Family Tennis Center in Mason, near Cincinnati, Ohio, USA, 23 August 2015. EPA/TANNEN MAURY

Novak Djokovic of Serbia (L) poses with Roger Federer of Switzerland after Federer defeated him in the final of the Western & Southern Open at the Linder Family Tennis Center in Mason, near Cincinnati, Ohio, USA, 23 August 2015. EPA/TANNEN MAURY

Roger Federer of Switzerland (C) poses with the ball boys and girls as he holds the winner’s trophy after defeating Novak Djokovic of Serbia in the final of the Western & Southern Open at the Linder Family Tennis Center in Mason, near Cincinnati, Ohio, USA, 23 August 2015. EPA/TANNEN MAURY