Serbia's Novak Djokovic returns the ball to Austria's Dominic Thiem during their ATP Madrid Open semi-final tennis match at the Caja Magica in Madrid on May 11, 2019. (Photo by JAVIER SORIANO / AFP via Getty Images)

Serbia's Novak Djokovic celebrates winning a point against Austria's Dominic Thiem during their ATP Madrid Open semi-final tennis match at the Caja Magica in Madrid on May 11, 2019. (Photo by JAVIER SORIANO / AFP via Getty Images)

Serbia's Novak Djokovic returns the ball to Austria's Dominic Thiem during their ATP Madrid Open semi-final tennis match at the Caja Magica in Madrid on May 11, 2019. (Photo by JAVIER SORIANO / AFP via Getty Images)

Serbia's Novak Djokovic serves to Austria's Dominic Thiem during their ATP Madrid Open semi-final tennis match at the Caja Magica in Madrid on May 11, 2019. (Photo by JAVIER SORIANO / AFP via Getty Images)

Austria's Dominic Thiem celebrates after winning a point against Serbia's Novak Djokovic during their ATP Madrid Open semi-final tennis match at the Caja Magica in Madrid on May 11, 2019. (Photo by JAVIER SORIANO / AFP via Getty Images)

Austria's Dominic Thiem reacts after losing a point to Serbia's Novak Djokovic during their ATP Madrid Open semi-final tennis match at the Caja Magica in Madrid on May 11, 2019. (Photo by JAVIER SORIANO / AFP via Getty Images)

Austria's Dominic Thiem reacts after losing a point to Serbia's Novak Djokovic during their ATP Madrid Open semi-final tennis match at the Caja Magica in Madrid on May 11, 2019. (Photo by JAVIER SORIANO / AFP via Getty Images)

Austria's Dominic Thiem serves to Serbia's Novak Djokovic during their ATP Madrid Open semi-final tennis match at the Caja Magica in Madrid on May 11, 2019. (Photo by JAVIER SORIANO / AFP via Getty Images)