Spain's Rafael Nadal returns the ball to Switzerland's Stan Wawrinka during their ATP Madrid Open quarter-final tennis match at the Caja Magica in Madrid on May 10, 2019. (Photo by OSCAR DEL POZO / AFP via Getty Images)

Spain's Rafael Nadal returns the ball to Switzerland's Stan Wawrinka during their ATP Madrid Open quarter-final tennis match at the Caja Magica in Madrid on May 10, 2019. (Photo by OSCAR DEL POZO / AFP via Getty Images)

Spain's Rafael Nadal returns the ball to Switzerland's Stan Wawrinka during their ATP Madrid Open quarter-final tennis match at the Caja Magica in Madrid on May 10, 2019. (Photo by OSCAR DEL POZO / AFP via Getty Images)

USA's Sloane Stephens returns the ball to Netherlands' Kiki Bertens during their WTA Madrid Open semi-final tennis match at the Caja Magica in Madrid on May 10, 2019. (Photo by OSCAR DEL POZO / AFP via Getty Images)

Spain's Rafael Nadal returns the ball to Switzerland's Stan Wawrinka during their ATP Madrid Open quarter-final tennis match at the Caja Magica in Madrid on May 10, 2019. (Photo by OSCAR DEL POZO / AFP via Getty Images)

USA's Sloane Stephens returns the ball to Netherlands' Kiki Bertens during their WTA Madrid Open semi-final tennis match at the Caja Magica in Madrid on May 10, 2019. (Photo by OSCAR DEL POZO / AFP via Getty Images)

USA's Sloane Stephens returns the ball to Netherlands' Kiki Bertens during their WTA Madrid Open semi-final tennis match at the Caja Magica in Madrid on May 10, 2019. (Photo by OSCAR DEL POZO / AFP via Getty Images)

USA's Sloane Stephens reacts as she plays against Netherlands' Kiki Bertens during their WTA Madrid Open semi-final tennis match at the Caja Magica in Madrid on May 10, 2019. (Photo by OSCAR DEL POZO / AFP via Getty Images)