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NEW YORK, NY - SEPTEMBER 09: Novak Djokovic of Serbia receives treatment on his shoulder during a break against Gael Monfils of France during their Men’s Singles Semifinal Match on Day Twelve of the 2016 US Open at the USTA Billie Jean King National Tennis Center on September 9, 2016 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Alex Goodlett/Getty Images)

NEW YORK, NY - SEPTEMBER 09:  Novak Djokovic of Serbia receives treatment on his shoulder during a break against Gael Monfils of France during their Men’s Singles Semifinal Match on Day Twelve of the 2016 US Open at the USTA Billie Jean King National Tennis Center on September 9, 2016 in the Flushing neighborhood of the Queens borough of New York City.  (Photo by Alex Goodlett/Getty Images)