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

Memphis Tennis

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

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