Ivo Karlović

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Kei Nishikori, of Japan, reacts after his return to Ivo Karlovic, of Croatia, in the singles final at the U.S. National Indoor Tennis Championships, Sunday, Feb. 16, 2014, in Memphis, Tenn. (AP Photo/Rogelio V. Solis)

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Ivo Karlovic, of Croatia, hits a return to Kei Nishikori, of Japan, in the singles final at the U.S. National Indoor Tennis Championships on Sunday, Feb. 16, 2014, in Memphis, Tenn. (AP Photo/Rogelio V. Solis)

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Kei Nishikori, of Japan, hits a return to Ivo Karlovic, of Croatia, in the singles final at the U.S. National Indoor Tennis Championships on Sunday, Feb. 16, 2014, in Memphis, Tenn. (AP Photo/Rogelio V. Solis)

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Ivo Karlovic, right, of Croatia, congratulates Kei Nishikori, of Japan, for winning the singles final 6-4, 7-6 (0) at the U.S. National Indoor Tennis Championships, Sunday, Feb. 16, 2014, in Memphis, Tenn. (AP Photo/Rogelio V. Solis)

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Ivo Karlovic, of Croatia, hits a backhand return to Kei Nishikori, of Japan, in the singles final at the U.S. National Indoor Tennis Championships on Sunday, Feb. 16, 2014, in Memphis, Tenn. (AP Photo/Rogelio V. Solis)

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Ivo Karlovic, of Croatia, serves to Kei Nishikori, of Japan, in the singles final at the U.S. National Indoor Tennis Championships on Sunday, Feb. 16, 2014, in Memphis, Tenn. (AP Photo/Rogelio V. Solis)

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Kei Nishikori, of Japan, hits a return to Ivo Karlovic, of Croatia, in the singles final at the U.S. National Indoor Tennis Championships on Sunday, Feb. 16, 2014, in Memphis, Tenn. (AP Photo/Rogelio V. Solis)

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Kei Nishikori, of Japan, hits a return to Ivo Karlovic, of Croatia, in the singles final at the U.S. National Indoor Tennis Championships on Sunday, Feb. 16, 2014, in Memphis, Tenn. (AP Photo/Rogelio V. Solis)

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Kei Nishikori,of Japan, hits a return to Ivo Karlovic, of Croatia, in the singles final at the U.S. National Indoor Tennis Championships on Sunday, Feb. 16, 2014, in Memphis, Tenn. (AP Photo/Rogelio V. Solis)

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Lu Yen-hsun. pf Taiwan, serves to Ivo Karlovic, of Croatia, during the semifinals of the U.S. National Indoor Tennis Championships on Saturday, Feb. 15, 2014, in Memphis, Tenn. Lu retired due to injury after losing the first set, (AP Photo/Rogelio V. Solis)

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Ivo Karlovic, of Croatia, serves to Lu Yen-hsun, of Taiwan, in the semifinals at the U.S. National Indoor Tennis Championships on Saturday, Feb. 15, 2014, in Memphis, Tenn. Karlovic won the first set 6-1, and the match after Lu retired due to injury. (AP Photo/Rogelio V. Solis)

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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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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)