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MELBOURNE, AUSTRALIA - JANUARY 23: Kei Nishikori of Japan serves in his quarter final match against Novak Djokovic of Serbia during day 10 of the 2019 Australian Open at Melbourne Park on January 23, 2019 in Melbourne, Australia. (Photo by Michael Dodge/Getty Images)

MELBOURNE, AUSTRALIA - JANUARY 23:  Kei Nishikori of Japan serves  in his quarter final match against Novak Djokovic of Serbia during day 10 of the 2019 Australian Open at Melbourne Park on January 23, 2019 in Melbourne, Australia.  (Photo by Michael Dodge/Getty Images)