David Goffin of Belgium hits a return shot to Novak Djokovic of Serbia during their third round match in the Western & Southern Open at the Linder Family Tennis Center in Mason, near Cincinnati, Ohio, USA, 20 August 2015. EPA/TANNEN MAURY

Ivo Karlovic of Croatia hits a return shot to Stan Wawrinka of Switzerland during their third round match in the Western & Southern Open at the Linder Family Tennis Center in Mason, Ohio, USA, 20 August 2015. EPA/TANNEN MAURY

Borna Coric of Croatia in action against Stan Wawrinka of Switzerland during their second round match at the Western & Southern Open at the Linder Family Tennis Center in Mason, near Cincinnati, Ohio, USA, 19 August 2015. EPA/TANNEN MAURY

Mardy Fish of the US hits a return shot to Andy Murray of Great Britain during their second round match in the Western and Southern Open at the Linder Family Tennis Center in Mason, Ohio, USA, 19 August 2015. EPA/TANNEN MAURY

Alexander Zverev of Germany in action against Borna Coric of Croatia during their first round match at the Western & Southern Open at the Linder Family Tennis Center in Mason, near Cincinnati, Ohio, USA, 18 August 2015. EPA/TANNEN MAURY