NEW YORK, NY - SEPTEMBER 11: Novak Djokovic of Serbia returns a shot to Marin Cilic of Croatia during their Men's Singles Semifinals match on Day Twelve of the 2015 US Open at the USTA Billie Jean King National Tennis Center on September 11, 2015 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Alex Goodlett/Getty Images)
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