LONDON, ENGLAND - NOVEMBER 16:  Novak Djokovic of Serbia smiles during the exhibition match against Andy Murray of Great Britain on day eight of the Barclays ATP World Tour Finals at O2 Arena on November 16, 2014 in London, England. The Singles final was replaced with and exhibition match after Roger Federer of Switzerland pulled out with an injury.  (Photo by Julian Finney/Getty Images)