Juan Martin Del Potro

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Roger Federer, of Switzerland, right, and Juan Martin del Potro, of Argentina, take a photo together before the start of a tennis match at the Miami Open, Monday, March 27, 2017 in Key Biscayne, Fla. (AP Photo/Wilfredo Lee)

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Roger Federer, of Switzerland, returns a shot from Juan Martin del Potro, of Argentina, during a tennis match at the Miami Open, Monday, March 27, 2017 in Key Biscayne, Fla. (AP Photo/Wilfredo Lee)

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Roger Federer, of Switzerland, returns a shot from Juan Martin del Potro, of Argentina, during a tennis match at the Miami Open, Monday, March 27, 2017 in Key Biscayne, Fla. (AP Photo/Wilfredo Lee)

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A tennis fan takes a photo during player introductions before a tennis match between Roger Federer, of Switzerland, and Juan Martin del Potro, of Argentina, at the Miami Open, Monday, March 27, 2017 in Key Biscayne, Fla. (AP Photo/Wilfredo Lee)

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Juan Martin del Potro, of Argentina, serves to Novak Djokovic, of Serbia, at the BNP Paribas Open tennis tournament, Tuesday, March 14, 2017, in Indian Wells, Calif. (AP Photo/Mark J. Terrill)

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Novak Djokovic, of Serbia, returns a shot to Juan Martin del Potro, of Argentina, at the BNP Paribas Open tennis tournament, Tuesday, March 14, 2017, in Indian Wells, Calif. (AP Photo/Mark J. Terrill)

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Somdev Devvarman of India returns the ball to Juan Martin Del Potro of Argentina during the second round of the Dubai Duty Free Tennis Championships in Dubai, United Arab Emirates, Tuesday, Feb. 25, 2014. (AP Photo/Kamran Jebreili)

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Juan Martin Del Potro of Argentina reacts after he lost a point against Somdev Devvarman of India during the second round of the Dubai Duty Free Tennis Championships in Dubai, United Arab Emirates, Tuesday, Feb. 25, 2014. (AP Photo/Kamran Jebreili)

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Juan Martin del Potro of Argentina waves after winning over Rhyne Williams of the U.S. in their first round match at the Australian Open tennis championship in Melbourne, Australia, Tuesday, Jan. 14, 2014. (AP Photo/Andrew Brownbill)

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Juan Martin del Potro of Argentina makes a backhand return to Rhyne Williams of the U.S. during their first round match at the Australian Open tennis championship in Melbourne, Australia, Tuesday, Jan. 14, 2014. (AP Photo/Andrew Brownbill)

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Juan Martin del Potro of Argentina serves to Rhyne Williams of the U.S. during their first round match at the Australian Open tennis championship in Melbourne, Australia, Tuesday, Jan. 14, 2014. (AP Photo/Andrew Brownbill)