NEW YORK, NY - SEPTEMBER 13:  Novak Djokovic of Serbia reacts against Roger Federer of Switzerland during their Men's Singles Final match on Day Fourteen of the 2015 US Open at the USTA Billie Jean King National Tennis Center on September 13, 2015 in the Flushing neighborhood of the Queens borough of New York City.  (Photo by Al Bello/Getty Images)