Jack Sock reacts to the serve of Ivo Karlovic, of Croatia, 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

Jack Sock reacts to the serve of Ivo Karlovic, of Croatia, 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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