Jack Sock, of the United States, hits a return to Ivo Karlovic, of Croatia, during the quarterfinals of the U.S. National Indoor Tennis Championships on Friday, Feb. 14, 2014 in Memphis, Tenn. (AP Photo/Rogelio V. Solis)

Memphis Tennis

Jack Sock, of the United States, hits a return to Ivo Karlovic, of Croatia, during the quarterfinals of the U.S. National Indoor Tennis Championships on Friday, Feb. 14, 2014 in Memphis, Tenn. (AP Photo/Rogelio V. Solis)

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