NEW YORK, NY - SEPTEMBER 03:  Stan Wawrinka of Switzerland returns a shot to Hyeon Chung of Korea during their Men's Singles Second Round match on Day Four of the 2015 US Open at the USTA Billie Jean King National Tennis Center on September 3, 2015 in the Flushing neighborhood of the Queens borough of New York City.  (Photo by Elsa/Getty Images)