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)

Greece's Stefanos Tsitsipas returns the ball to 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)

Greece's Stefanos Tsitsipas returns the ball to 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)

Greece's Stefanos Tsitsipas returns the ball to 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)

Greece's Stefanos Tsitsipas returns the ball to 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)

Greece's Stefanos Tsitsipas returns the ball to 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)