NEW YORK, NY - SEPTEMBER 07: Novak Djokovic of Serbia returns the ball during his men's singles semi-final match against Kei Nishikori of Japan on Day Twelve of the 2018 US Open at the USTA Billie Jean King National Tennis Center on September 7, 2018 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Jaime Lawson/Getty Images for USTA)