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NEW YORK, NY - AUGUST 30: Gael Monfils of France reacts to an injury during the men's singles second round match against Kei Nishikori of Japan on Day Four of the 2018 US Open at the USTA Billie Jean King National Tennis Center on August 30, 2018 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Sarah Stier/Getty Images)

NEW YORK, NY - AUGUST 30:  Gael Monfils of France reacts to an injury during the men's singles second round match against Kei Nishikori of Japan on Day Four of the 2018 US Open at the USTA Billie Jean King National Tennis Center on August 30, 2018 in the Flushing neighborhood of the Queens borough of New York City.  (Photo by Sarah Stier/Getty Images)