Spain’s Rafael Nadal in action against Serbia’s Novak Djokovic during their semi final match at the ATP Tour tennis finals tournament at the O2 Arena in London, Britain, 21 November 2015. EPA/WILL OLIVER

Switzerland’s Roger Federer returns the ball to his compatriot Stanislas Wawrinka during their semi final match of the ATP World Tour Finals tennis tournament in London, Britain, 21 November 2015. EPA/ANDY RAIN

Germany’s Alexander Zverev receives the ATP Star of Tomorrow Award at the ATP Tour tennis finals tournament at the O2 Arena in London, Britain, 21 November 2015. EPA/WILL OLIVER

Serbia’s Novak Djokovic celebrates following his two set win over Spain’s Rafael Nadal during their semi final match at the ATP Tour tennis finals tournament at the O2 Arena in London, Britain, 21 November 2015. EPA/ANDY RAIN

Switzerland’s Roger Federer returns the ball to his compatriot Stanislas Wawrinka during their semi final match of the ATP World Tour Finals tennis tournament in London, Britain, 21 November 2015. EPA/ANDY RAIN

Switzerland’s Stanislas Wawrinka returns the ball to his compatriot Roger Federer during their semi final match of the ATP World Tour Finals tennis tournament in London, Britain, 21 November 2015. EPA/ANDY RAIN