Novak Djokovic of Serbia reacts after winning his semi-final tennis match against Kei Nishikori of Japan at the ATP World Tour Finals at the O2 Arena in London November 15, 2014. REUTERS/Toby Melville
Novak Djokovic of Serbia reacts after winning his semi-final tennis match against Kei Nishikori of Japan at the ATP World Tour Finals at the O2 Arena in London November 15, 2014. REUTERS/Toby Melville