TOPSHOT - Austria's Dominic Thiem returns the ball 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)

TOPSHOT - 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)

TOPSHOT - 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)

TOPSHOT - 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)

TOPSHOT - 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)

TOPSHOT - Germany's Alexander Zverev serves to Greece's Stefanos Tsitsipas during their ATP Madrid Open quarter-final tennis match at the Caja Magica in Madrid on May 10, 2019. (Photo by OSCAR DEL POZO / AFP via Getty Images)

TOPSHOT - Austria's Dominic Thiem plays against Switzerland's Roger Federer during their ATP Madrid Open quarter-final tennis match at the Caja Magica in Madrid on May 10, 2019. (Photo by GABRIEL BOUYS / AFP via Getty Images)

TOPSHOT - Switzerland's Roger Federer serves to Austria's Dominic Thiem during their ATP Madrid Open quarter-final tennis match at the Caja Magica in Madrid on May 10, 2019. (Photo by GABRIEL BOUYS / AFP via Getty Images)