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)

Netherlands' Kiki Bertens returns the ball to USA's Sloane Stephens during the 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)

Netherlands' Kiki Bertens serves to USA's Sloane Stephens during the 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)

Netherlands' Kiki Bertens returns the ball to USA's Sloane Stephens during the 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)

Netherlands' Kiki Bertens reacts after winning a point against USA's Sloane Stephens during the 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)

Netherlands' Kiki Bertens reacts after winning a point against USA's Sloane Stephens during the 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)

Germany's Alexander Zverev returns the ball to Greece's Stefanos Tsitsipas 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)