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MELBOURNE, AUSTRALIA - JANUARY 17: Kei Nishikori of Japan plays a backhand in his second round match against Ivo Karlovic of Croatia during day four of the 2019 Australian Open at Melbourne Park on January 17, 2019 in Melbourne, Australia. (Photo by Fred Lee/Getty Images)

MELBOURNE, AUSTRALIA - JANUARY 17:  Kei Nishikori of Japan plays a backhand in his second round match against Ivo Karlovic of Croatia during day four of the 2019 Australian Open at Melbourne Park on January 17, 2019 in Melbourne, Australia. (Photo by Fred Lee/Getty Images)