Novak Djokovic of Serbia reacts to a game win against Andy Murray of Britain during their ATP World Tour Finals singles tennis match at the O2 Arena in London, Wednesday, Nov. 7, 2012. (AP Photo/Sang Tan)

Photo by: Sang Tan

Novak Djokovic of Serbia reacts to a game win against Andy Murray of Britain during their ATP World Tour Finals singles tennis match at the O2 Arena in London, Wednesday, Nov. 7, 2012. (AP Photo/Sang Tan)