Jack Sock

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Jiri Vesely, of the Czech Republic, watches his return to Jack Sock, of the United States, at the U.S. National Indoor Tennis Championships on Wednesday, Feb. 12, 2014, in Memphis, Tenn. (AP Photo/Rogelio V. Solis)

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Jack Sock reaches to hit a return to Jiri Vesely, of Czech Republic, in the second round match at the U.S. National Indoor Tennis Championships on Wednesday, Feb. 12, 2014, in Memphis, Tenn. (AP Photo/Rogelio V. Solis)

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Jack Sock watches his return to Jiri Vesely, of Czech Republic, in the second round match at the U.S. National Indoor Tennis Championships on Wednesday, Feb. 12, 2014, in Memphis, Tenn. (AP Photo/Rogelio V. Solis)

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Jack Sock hits the ball during his match against Igor Sijsling, from the Netherlands, at the Citi Open tennis tournament, Tuesday, July 30, 2013 in Washington. Sock won 6-4, 6-2. (AP Photo/Alex Brandon)

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**FILE** Jack Sock returns a shot to Feliciano Lopez, of Spain, in a quarterfinal round tennis match at the U.S. National Indoor Championships on Friday, Feb. 22, 2013, in Memphis, Tenn. (AP Photo/Mark Humphrey)

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**FILE** Jack Sock, left, of Lincoln, Neb., passes Andy Roddick, right, of Omaha, Neb., during their match at the U.S. Open tennis tournament in New York, Friday, Sept. 2, 2011. (AP Photo/Charles Krupa)

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Melanie Oudin and Jack Sock hold the championship trophy after winning the mixed doubles at the U.S. Open tennis tournament in New York, Friday, Sept. 9, 2011. Oudin and Sock defeated Gisela Dulko of Argentina and Eduardo Schwank of Argentina. (AP Photo/Matt Slocum)

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Andy Roddick, of the United States, acknowledges the crowd after defeating Jack Sock, also of the United States, 6-3, 6-3, 6-4 during the U.S. Open tennis tournament in New York, Friday, Sept. 2, 2011. (AP Photo/Charles Krupa)