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 OSCAR DEL POZO / 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 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 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 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 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 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)

France's Jeremy Chardy returns the ball to Serbia's Novak Djokovic 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)