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NEW YORK, NY - SEPTEMBER 02: Jack Sock of the United States returns a shot to Marin Cilic of Croatia during his third round Men's Singles match on Day Five of the 2016 US Open at the USTA Billie Jean King National Tennis Center on September 2, 2016 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Al Bello/Getty Images)

NEW YORK, NY - SEPTEMBER 02:  Jack Sock of the United States returns a shot to Marin Cilic of Croatia during his third round Men's Singles match on Day Five of the 2016 US Open at the USTA Billie Jean King National Tennis Center on September 2, 2016 in the Flushing neighborhood of the Queens borough of New York City.  (Photo by Al Bello/Getty Images)