Switzerland’s Roger Federer returns the ball to Tomas Berdych of Czech Republic during their match at the ATP World Tour Finals in London, Britain, 15 November 2015. EPA/WILL OLIVER

Tomas Berdych of Czech Republic enters the arena before his match against Switzerland’s Roger Federer during the ATP World Tour Finals in London, Britain, 15 November 2015. EPA/WILL OLIVER

Japan’s Kei Nishikori in action against Serbia’s Novak Djokovic during their match at the ATP World Tour Finals in London, Britain, 15 November 2015. EPA/WILL OLIVER

The O2 arena is illuminated in the French tricolor during the ATP World Tennis Finals in London, Britain, 15 November 2015. At least 129 people were killed in a series of terrorist attacks in Paris, France on 13 November 2015. EPA/WILL OLIVER

Switzerland’s Roger Federer returns the ball to Tomas Berdych of Czech Republic during their match at the ATP World Tour Finals in London, Britain, 15 November 2015. EPA/WILL OLIVER