NEW YORK, NEW YORK - SEPTEMBER 01: Trainer Paul Ness tends to Novak Djokovic of Serbia during his Men's Singles fourth round match against Stan Wawrinka of Switzerland on day seven of the 2019 US Open at the USTA Billie Jean King National Tennis Center on September 01, 2019 in Queens borough of New York City. (Photo by Matthew Stockman/Getty Images)
NEW YORK, NEW YORK - SEPTEMBER 01: Stan Wawrinka of Switzerland returns a shot during his Men's Singles fourth round match against Novak Djokovic of Serbia on day seven of the 2019 US Open at the USTA Billie Jean King National Tennis Center on September 01, 2019 in Queens borough of New York City. (Photo by Matthew Stockman/Getty Images)
NEW YORK, NEW YORK - SEPTEMBER 01: Novak Djokovic (R) of Serbia embraces Stan Wawrinka (L) of Switzerland after Djokovic retired due to a shoulder injury during their Men's Singles fourth round match on day seven of the 2019 US Open at the USTA Billie Jean King National Tennis Center on September 01, 2019 in Queens borough of New York City. (Photo by Matthew Stockman/Getty Images)
NEW YORK, NEW YORK - SEPTEMBER 01: Novak Djokovic of Serbia returns a shot during his Men's Singles fourth round match against Stan Wawrinka of Switzerland on day seven of the 2019 US Open at the USTA Billie Jean King National Tennis Center on September 01, 2019 in Queens borough of New York City. (Photo by Matthew Stockman/Getty Images)
