Greece's Stefanos Tsitsipas talks to the umpire during the ATP Madrid Open semi-final tennis match against Spain's Rafael Nadal at the Caja Magica in Madrid on May 11, 2019. (Photo by OSCAR DEL POZO / AFP via Getty Images)

Greece's Stefanos Tsitsipas celebrates winning a point against Spain's Rafael Nadal during their ATP Madrid Open semi-final tennis match at the Caja Magica in Madrid on May 11, 2019. (Photo by OSCAR DEL POZO / AFP via Getty Images)

Spain's Rafael Nadal returns the ball to Greece's Stefanos Tsitsipas during their ATP Madrid Open semi-final tennis match at the Caja Magica in Madrid on May 11, 2019. (Photo by OSCAR DEL POZO / AFP via Getty Images)

Spain's Rafael Nadal returns the ball to Greece's Stefanos Tsitsipas during their ATP Madrid Open semi-final tennis match at the Caja Magica in Madrid on May 11, 2019. (Photo by OSCAR DEL POZO / AFP via Getty Images)

Spain's Rafael Nadal returns the ball to Greece's Stefanos Tsitsipas during their ATP Madrid Open semi-final tennis match at the Caja Magica in Madrid on May 11, 2019. (Photo by OSCAR DEL POZO / AFP via Getty Images)

Spain's Rafael Nadal returns the ball to Greece's Stefanos Tsitsipas during their ATP Madrid Open semi-final tennis match at the Caja Magica in Madrid on May 11, 2019. (Photo by OSCAR DEL POZO / AFP via Getty Images)

Spain's Rafael Nadal returns the ball to Greece's Stefanos Tsitsipas during their ATP Madrid Open semi-final tennis match at the Caja Magica in Madrid on May 11, 2019. (Photo by OSCAR DEL POZO / AFP via Getty Images)

Spain's Rafael Nadal celebrates winning a point against Greece's Stefanos Tsitsipas during their ATP Madrid Open semi-final tennis match at the Caja Magica in Madrid on May 11, 2019. (Photo by OSCAR DEL POZO / AFP via Getty Images)