LONDON, ENGLAND - NOVEMBER 12: Novak Djokovic of Serbia celebrates match point during his men's singles final match against Roger Federer of Switzerland during day eight of the ATP World Tour Finals at O2 Arena on November 12, 2012 in London, England. (Photo by Matthew Lewis/Getty Images)

Novak Djokovic speaks with CNN's Alex Thomas after winning the ATP World Tour Final.

Novak Djokovic speaks with CNN's Alex Thomas after winning the ATP World Tour Final.