Juan Martin del Potro of Argentina plays a forehand shot in his quarterfinals match against Radek Stepanek of the Czech Republic during the Sydney International Tennis Tournament in Sydney, Australia, Thursday, Jan. 9, 2014. (AP Photo/Rob Griffith)

Australia Tennis Sydney International

Juan Martin del Potro of Argentina plays a forehand shot in his quarterfinals match against Radek Stepanek of the Czech Republic during the Sydney International Tennis Tournament in Sydney, Australia, Thursday, Jan. 9, 2014. (AP Photo/Rob Griffith)

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