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)

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)

Greece's Stefanos Tsitsipas reacts as he plays against Germany's Alexander Zverev 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)

Germany's Alexander Zverev reacts as he plays against 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)

Germany's Alexander Zverev reacts as he plays against 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)

Germany's Alexander Zverev plays against 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)

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)