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NEW YORK, NY - SEPTEMBER 03: Jack Sock of the United States serves to Ruben Bemelmans of Belgium during their Men's Singles Second Round match on Day Four of the 2015 US Open at the USTA Billie Jean King National Tennis Center on September 3, 2015 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Al Bello/Getty Images)

NEW YORK, NY - SEPTEMBER 03:  Jack Sock of the United States serves to Ruben Bemelmans of Belgium during their Men's Singles Second Round match on Day Four of the 2015 US Open at the USTA Billie Jean King National Tennis Center on September 3, 2015 in the Flushing neighborhood of the Queens borough of New York City.  (Photo by Al Bello/Getty Images)