Spain's Rafael Nadal returns the ball to US Frances Tiafoe 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)

Serbia's Novak Djokovic reacts in front of France's Jeremy Chardy 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)

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)

France's Jeremy Chardy reacts in front of 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)

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

Serbia's Novak Djokovic serves to France's Jeremy Chardy 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)

Serbia's Novak Djokovic celebrates after defeating France's Jeremy Chardy 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 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)