NEW YORK, NY - SEPTEMBER 04:  Serena Williams of the United States returns a shot to Bethanie Mattek-Sands of the United States during their Women's Singles Third Round match on Day Five of the 2015 US Open at the USTA Billie Jean King National Tennis Center on September 4, 2015 in the Flushing neighborhood of the Queens borough of New York City.  (Photo by Elsa/Getty Images)