Serbia's Novak Djokovic returns the ball to Greece's Stefanos Tsitsipas during their ATP Madrid Open final tennis match at the Caja Magica in Madrid on May 12, 2019. (Photo by JAVIER SORIANO / AFP via Getty Images)

Greece's Stefanos Tsitsipas returns the ball to Serbia's Novak Djokovic during their ATP Madrid Open final tennis match at the Caja Magica in Madrid on May 12, 2019. (Photo by OSCAR DEL POZO / AFP via Getty Images)

TOPSHOT - Serbia's Novak Djokovic serves to Greece's Stefanos Tsitsipas during their ATP Madrid Open final tennis match at the Caja Magica in Madrid on May 12, 2019. (Photo by OSCAR DEL POZO / AFP via Getty Images)

Serbia's Novak Djokovic returns the ball to Greece's Stefanos Tsitsipas during their ATP Madrid Open final tennis match at the Caja Magica in Madrid on May 12, 2019. (Photo by JAVIER SORIANO / AFP via Getty Images)

Serbia's Novak Djokovic returns the ball to Greece's Stefanos Tsitsipas during their ATP Madrid Open final tennis match at the Caja Magica in Madrid on May 12, 2019. (Photo by JAVIER SORIANO / AFP via Getty Images)

Greece's Stefanos Tsitsipas returns the ball to Serbia's Novak Djokovic during their ATP Madrid Open final tennis match at the Caja Magica in Madrid on May 12, 2019. (Photo by JAVIER SORIANO / AFP via Getty Images)

Serbia's Novak Djokovic arrives to play the ATP Madrid Open final tennis match against Greece's Stefanos Tsitsipas at the Caja Magica in Madrid on May 12, 2019. (Photo by JAVIER SORIANO / AFP via Getty Images)

Serbia's Novak Djokovic looks on before the ATP Madrid Open final tennis match against Greece's Stefanos Tsitsipas at the Caja Magica in Madrid on May 12, 2019. (Photo by OSCAR DEL POZO / AFP via Getty Images)