Horia Tecau (L) celebrates with doubles partner Jean-Julien Rojer (R) after winning the doubles final match at the ATP Tour tennis finals tournament at the O2 Arena in London, Britain, 22 November 2015. EPA/ANDY RAIN

Serbia’s Novak Djokovic lifts the ATP Tour Finals trophy following his two sets win over Switzerland’s Roger Federer at the ATP Tour tennis finals tournament at the O2 Arena in London, Britain, 22 November 2015. EPA/ANDY RAIN

Serbia’s Novak Djokovic (L) with the winner’s trophy following his two sets win over Switzerland’s Roger Federer (R) at the ATP Tour tennis finals tournament at the O2 Arena in London, Britain, 22 November 2015. EPA/ANDY RAIN

Serbia’s Novak Djokovic with the ATP Tour Finals trophy following his two sets win over Switzerland’s Roger Federer at the ATP Tour tennis finals tournament at the O2 Arena in London, Britain, 22 November 2015. EPA/ANDY RAIN EPA/ANDY RAIN

Novak Djokovic of Serbia celebrates with the trophy after winning against Roger Federer of Switzerland in the final at the ATP World Tour Finals in London, Britain, 22 November 2015. EPA/ANDY RAIN