NEW YORK, NY - AUGUST 29: Serena Williams of the United States serves against Carina Witthoeft of Germany during her women's singles second round match on Day Three of the 2018 US Open at the USTA Billie Jean King National Tennis Center on August 29, 2018 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Jaime Lawson/Getty Images for USTA)