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STUTTGART, GERMANY - JUNE 10: Juan Martin del Potro of Argentinia returns against Gilles Simon of France during the quarterfinals on day 7 of Mercedes Cup 2016 on June 10, 2016 in Stuttgart, Germany. (Photo by Daniel Kopatsch/Bongarts/Getty Images)

STUTTGART, GERMANY - JUNE 10: Juan Martin del Potro of Argentinia returns against Gilles Simon of France during the quarterfinals on day 7 of Mercedes Cup 2016 on June 10, 2016 in Stuttgart, Germany. (Photo by Daniel Kopatsch/Bongarts/Getty Images)