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)

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)

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)

Switzerland's Roger Federer serves to Austria's Dominic Thiem during their ATP Madrid Open quarter-final tennis match at the Caja Magica in Madrid on May 10, 2019. (Photo by GABRIEL BOUYS / AFP via Getty Images)