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 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)

Switzerland's Roger Federer reacts as he plays against 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)

Switzerland's Roger Federer returns the ball 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)

Romania's Simona Halep celebrates defeating Switzerland's Belinda Bencic during their WTA Madrid Open semi-final tennis match at the Caja Magica in Madrid on May 10, 2019. (Photo by GABRIEL BOUYS / AFP via Getty Images)

Romania's Simona Halep blows a kiss to celebrate defeating Switzerland's Belinda Bencic during their WTA Madrid Open semi-final tennis match at the Caja Magica in Madrid on May 10, 2019. (Photo by GABRIEL BOUYS / AFP via Getty Images)

Romania's Simona Halep returns the ball to Switzerland's Belinda Bencic during their WTA Madrid Open semi-final tennis match at the Caja Magica in Madrid on May 10, 2019. (Photo by GABRIEL BOUYS / AFP via Getty Images)

Romania's Simona Halep returns the ball to Switzerland's Belinda Bencic during their WTA Madrid Open semi-final tennis match at the Caja Magica in Madrid on May 10, 2019. (Photo by GABRIEL BOUYS / AFP via Getty Images)