LONDON, ENGLAND - NOVEMBER 17:  Novak Djokovic coach Boris Becker (C), Alec Stewart (L) and Marian Vajda (R) watch on during the match between Novak Djokovic of Serbia and Roger Federer of Switzerland during day three of the Barclays ATP World Tour Finals at the O2 Arena on November 17, 2015 in London, England.  (Photo by Julian Finney/Getty Images)