NEW YORK, NY - SEPTEMBER 09:  Novak Djokovic of Serbia reacts during a break in his men's singles final match against Rafael Nadal of Spain on Day Fifteen of the 2013 US Open at the USTA Billie Jean King National Tennis Center on September 9, 2013 in the Flushing neighborhood of the Queens borough of New York City.  (Photo by Matthew Stockman/Getty Images)