NEW YORK, NEW YORK - AUGUST 30: Roger Federer of Switzerland serves the ball during his Men's Singles third round match against Daniel Evans of Great Britain on day five of the 2019 US Open at the USTA Billie Jean King National Tennis Center on August 30, 2019 in Queens borough of New York City. The US Open draws established and up-and-coming tennis talent from around the world to Queens, New York City every August, and thousands of fans. The tournament last two weeks, and is one of four Grand Slam tennis tournaments annually. (Photo by Katharine Lotze/Getty Images)
NEW YORK, NEW YORK - AUGUST 31: Hyeon Chung of South Korea serves the ball during his Men's Singles third round match against Rafael Nadal of Spain on day six of the 2019 US Open at the USTA Billie Jean King National Tennis Center on August 31, 2019 in Queens borough of New York City. (Photo by Katharine Lotze/Getty Images)
NEW YORK, NEW YORK - AUGUST 28: Serena Williams of the United States looks on during her Women's Singles second round match against Catherine McNally of the United States on day three of the 2019 US Open at the USTA Billie Jean King National Tennis Center on August 28, 2019 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Katharine Lotze/Getty Images)
NEW YORK, NEW YORK - AUGUST 31: Naomi Osaka of Japan reacts during her Women's Singles third round match against Cori Gauff of the United States on day six of the 2019 US Open at the USTA Billie Jean King National Tennis Center on August 31, 2019 in Queens borough of New York City. (Photo by Katharine Lotze/Getty Images)
NEW YORK, NEW YORK - AUGUST 31: Cori Gauff of the United States serves during her Women's Singles third round match against Naomi Osaka of Japan on day six of the 2019 US Open at the USTA Billie Jean King National Tennis Center on August 31, 2019 in Queens borough of New York City. (Photo by Katharine Lotze/Getty Images)
NEW YORK, NEW YORK - AUGUST 31: Rafael Nadal of Spain serves during his Men's Singles third round match against Hyeon Chung of South Korea on day six of the 2019 US Open at the USTA Billie Jean King National Tennis Center on August 31, 2019 in Queens borough of New York City. (Photo by Katharine Lotze/Getty Images)
NEW YORK, NEW YORK - AUGUST 31: Cori Gauff of the United States reacts during her Women's Singles third round match against Naomi Osaka of Japan on day six of the 2019 US Open at the USTA Billie Jean King National Tennis Center on August 31, 2019 in Queens borough of New York City. (Photo by Katharine Lotze/Getty Images)
NEW YORK, NEW YORK - AUGUST 28: Novak Djokovic of Serbia returns a shot during his Men's Singles second round match against Juan Ignacio Londero of Argentina on day three of the 2019 US Open at the USTA Billie Jean King National Tennis Center on August 28, 2019 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Katharine Lotze/Getty Images)
NEW YORK, NEW YORK - AUGUST 30: Denis Kudla of the United States returns a shot during his Men's Singles third round match against Novak Djokovic of Serbia on day five of the 2019 US Open at the USTA Billie Jean King National Tennis Center on August 30, 2019 in Queens borough of New York City. (Photo by Katharine Lotze/Getty Images)
NEW YORK, NEW YORK - AUGUST 31: Rafael Nadal of Spain returns a shot during his Men's Singles third round match against Hyeon Chung of South Korea on day six of the 2019 US Open at the USTA Billie Jean King National Tennis Center on August 31, 2019 in Queens borough of New York City. (Photo by Katharine Lotze/Getty Images)
NEW YORK, NEW YORK - AUGUST 28: Serena Williams of the United States reacts during her Women's Singles second round match against Catherine McNally of the United States on day three of the 2019 US Open at the USTA Billie Jean King National Tennis Center on August 28, 2019 in the Flushing neighborhood of the Queens borough of New York City. (Photo by Katharine Lotze/Getty Images)
NEW YORK, NEW YORK - AUGUST 31: Cori Gauff of the United States looks on during her Women's Singles third round match against Naomi Osaka of Japan on day six of the 2019 US Open at the USTA Billie Jean King National Tennis Center on August 31, 2019 in Queens borough of New York City. (Photo by Katharine Lotze/Getty Images)