Tomas Berdych of the Czech Republic returns the ball to Japan’s Kei Nishikori during their round robin match at the ATP World Tour Finals tennis tournament in London, Britain, 17 November 2015. EPA/WILL OLIVER

Spain’s Rafael Nadal serves to Britain’s Andy Murray during their round robin match of the ATP World Tour Finals tennis tournament in London, Britain, 18 November 2015. EPA/FACUNDO ARRIZABALAGA

Supporters of Japan’s Kei Nishikori cheer during Nishikori’s round robin match against Tomas Berdych of the Czech Republic at the ATP World Tour Finals tennis tournament in London, Britain, 17 November 2015. EPA/WILL OLIVER

Switzerland’s Roger Federer waves after winning against Serbia’s Novak Djokovic during their round robin game during the ATP World Tour Finals in London, Britain, 17 November 2015. EPA/WILL OLIVER

Britain’s Andy Murray returns the ball to Spain’s Rafael Nadal during their round robin match of the ATP World Tour Finals tennis tournament in London, Britain, 18 November 2015. EPA/FACUNDO ARRIZABALAGA