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)

Greece's Stefanos Tsitsipas serves 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 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 OSCAR DEL POZO / AFP via Getty Images)

Greece's Stefanos Tsitsipas returns the ball against 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)

Greece's Stefanos Tsitsipas returns the ball against 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)

Greece's Stefanos Tsitsipas celebrates winning a point against 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)