NEW YORK, NY - SEPTEMBER 07: Kei Nishikori of Japan returns the ball during his men's singles semi-final match against Novak Djokovic of Serbia 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)