Jack Sock, of the United States, reaches to return a serve by Croatia's Ivo Karlovic 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, reaches to return a serve by Croatia's Ivo Karlovic 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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