NEW YORK, NY - SEPTEMBER 07: Rafael Nadal of Spain returns the ball during his men's singles semi-final match against Juan Martin del Potro of Argentina 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)