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Guido Pella of Argentina returns a shot during the quarterfinal of ATP Mercedes Cup tennis tournament against Roger Federer of Switzerland in Stuttgart, Germany on June 15, 2018. Guido Pella lost 0-2. (Xinhua/Philippe Ruiz)

Guido Pella of Argentina returns a shot during the quarterfinal of ATP Mercedes Cup tennis tournament against Roger Federer of Switzerland in Stuttgart, Germany on June 15, 2018. Guido Pella lost 0-2. (Xinhua/Philippe Ruiz)