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MELBOURNE, AUSTRALIA - JANUARY 28: Kei Nishikori of Japan plays a backhand in his quarterfinal match against Stanislas Wawrinka of Switzerland during day 10 of the 2015 Australian Open at Melbourne Park on January 28, 2015 in Melbourne, Australia. (Photo by Cameron Spencer/Getty Images)

MELBOURNE, AUSTRALIA - JANUARY 28:  Kei Nishikori of Japan plays a backhand in his quarterfinal match against Stanislas Wawrinka of Switzerland during day 10 of the 2015 Australian Open at Melbourne Park on January 28, 2015 in Melbourne, Australia.  (Photo by Cameron Spencer/Getty Images)