France's Gael Monfils serves the ball to Switzerland's Roger Federer 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)

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 OSCAR DEL POZO / 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 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 Belinda Bencic serves the ball to Japan's Naomi Osaka during their WTA Madrid Open round of 8 tennis match at the Caja Magica in Madrid on May 9, 2019. (Photo by GABRIEL BOUYS / AFP via Getty Images)

Japan's Naomi Osaka returns the ball to Switzerland's Belinda Bencic during their WTA Madrid Open round of 8 tennis match at the Caja Magica in Madrid on May 9, 2019. (Photo by GABRIEL BOUYS / AFP via Getty Images)

Japan's Naomi Osaka serves the ball to Switzerland's Belinda Bencic during their WTA Madrid Open round of 8 tennis match at the Caja Magica in Madrid on May 9, 2019. (Photo by GABRIEL BOUYS / AFP via Getty Images)

Japan's Naomi Osaka serves the ball to Switzerland's Belinda Bencic during their WTA Madrid Open round of 8 tennis match at the Caja Magica in Madrid on May 9, 2019. (Photo by GABRIEL BOUYS / AFP via Getty Images)