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)

US Frances Tiafoe arrives for his ATP Madrid Open round of 16 tennis match against Spain's Rafael Nadal at the Caja Magica in Madrid on May 9, 2019. (Photo by OSCAR DEL POZO / AFP via Getty Images)

The shoes of Spain's Rafael Nadal are pictured during their ATP Madrid Open round of 16 tennis match against US Frances Tiafoe 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)

US Frances Tiafoe reacts during an ATP Madrid Open round of 16 tennis match against Spain's Rafael Nadal at the Caja Magica in Madrid on May 9, 2019. (Photo by OSCAR DEL POZO / AFP via Getty Images)

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

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

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