Switzerland’s Roger Federer returns to Japan’s Kei Nishikori during a singles group match during the ATP Tour tennis finals tournament at the O2 Arena in London, Britain, 19 November 2015. EPA/ANDY RAIN

Britain’s Andy Murray trains ahead of his match wtth Switzerland’s Stanilas Wawrinka on Friday in a singles group match at the ATP Tour tennis finals tournament at the O2 Arena in London, Britain, 19 November 2015. EPA/ANDY RAIN

Serbia’s Novak Djokovic carries a huge training ball during a training session ahead of his match with Tomas Berdych during the ATP Tour tennis finals tournament at the O2 Arena in London, Britain, 19 November 2015. EPA/ANDY RAIN

Japan’s Kei Nishikori returns to Switzerland’s Roger Federer during a singles group match during the ATP Tour tennis finals tournament at the O2 Arena in London, Britain, 19 November 2015. EPA/ANDY RAIN

Switzerland’s Roger Federer returns to Japan’s Kei Nishikori during a singles group match during the ATP Tour tennis finals tournament at the O2 Arena in London, Britain, 19 November 2015. EPA/ANDY RAIN