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)

Spain's Rafael Nadal celebrates winning a point against 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)

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)

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)

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)

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)

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)

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)