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Stefanos Tsitsipas of Greece returns a ball to Joao Sousa of Portugal during the Millennium Estoril Open ATP 250 tennis tournament semifinal, at the Clube de Tenis do Estoril in Estoril, Portugal on May 5, 2018. (Joao Sousa won 6-4;1-6;7-6) ( Photo by Pedro Fiúza/NurPhoto via Getty Images)

Stefanos Tsitsipas of Greece returns a ball to Joao Sousa of Portugal during the Millennium Estoril Open ATP 250 tennis tournament semifinal, at the Clube de Tenis do Estoril in Estoril, Portugal on May 5, 2018. (Joao Sousa won 6-4;1-6;7-6) ( Photo by Pedro Fiúza/NurPhoto via Getty Images)