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LONDON, ENGLAND - NOVEMBER 22: A dejected Roger Federer (R) of Switzerland shakes hands with Novak Djokovic of Serbia following his defeat during the men's singles final on day eight of the Barclays ATP World Tour Finals at the O2 Arena on November 22, 2015 in London, England. (Photo by Julian Finney/Getty Images)

LONDON, ENGLAND - NOVEMBER 22:  A dejected Roger Federer (R) of Switzerland shakes hands with Novak Djokovic of Serbia following his defeat during the men's singles final on day eight of the Barclays ATP World Tour Finals at the O2 Arena on November 22, 2015 in London, England.  (Photo by Julian Finney/Getty Images)