Switzerland's Roger Federer returns the ball to France's Gael Monfils during their ATP Madrid Open round of 16 tennis match at the Caja Magica in Madrid on May 9, 2019. (Photo by GABRIEL BOUYS / AFP via Getty Images)

Switzerland's Roger Federer celebrates after defeating France's Gael Monfils during their ATP Madrid Open round of 16 tennis match at the Caja Magica in Madrid on May 9, 2019. (Photo by GABRIEL BOUYS / AFP via Getty Images)

Switzerland's Roger Federer serves to France's Gael Monfils during their ATP Madrid Open round of 16 tennis match against Switzerland's Roger Federer at the Caja Magica in Madrid on May 9, 2019. (Photo by GABRIEL BOUYS / AFP via Getty Images)

Australia's Ashleigh Barty returns the ball to Romania's Simona Halep during their WTA Madrid Open round of 8 tennis match at the Caja Magica in Madrid on May 9, 2019. Two-time champion Simona Halep overcame Ashleigh Barty 7-5, 7-5 to reach her fourth Madrid Open semi-final today. Romania's third seed and former number one will next play the winner from top seed Naomi Osaka and on-form Swiss Belinda Bencic. (Photo by GABRIEL BOUYS / AFP via Getty Images)

Spain's Rafael Nadal runs for the ball against Greece's Stefanos Tsitsipas during their ATP Madrid Open semi-final tennis match at the Caja Magica in Madrid on May 11, 2019. (Photo by OSCAR DEL POZO / AFP via Getty Images)

Greece's Stefanos Tsitsipas returns the ball to Spain's Rafael Nadal during their ATP Madrid Open semi-final tennis match at the Caja Magica in Madrid on May 11, 2019. (Photo by OSCAR DEL POZO / AFP via Getty Images)

Greece's Stefanos Tsitsipas celebrates winning a point against Spain's Rafael Nadal during their ATP Madrid Open semi-final tennis match at the Caja Magica in Madrid on May 11, 2019. (Photo by OSCAR DEL POZO / AFP via Getty Images)

Greece's Stefanos Tsitsipas celebrates winning a point against Spain's Rafael Nadal during their ATP Madrid Open semi-final tennis match at the Caja Magica in Madrid on May 11, 2019. (Photo by OSCAR DEL POZO / AFP via Getty Images)