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A court view of Japan's Kei Nishikori (R, in red) playing against Switzerland's Roger Federer during their men's singles fourth round match on day seven of the Australian Open tennis tournament in Melbourne on January 22, 2017. / AFP / WILLIAM WEST / IMAGE RESTRICTED TO EDITORIAL USE - STRICTLY NO COMMERCIAL USE (Photo credit should read WILLIAM WEST/AFP via Getty Images)

A court view of Japan's Kei Nishikori (R, in red) playing against Switzerland's Roger Federer during their men's singles fourth round match on day seven of the Australian Open tennis tournament in Melbourne on January 22, 2017. / AFP / WILLIAM WEST / IMAGE RESTRICTED TO EDITORIAL USE - STRICTLY NO COMMERCIAL USE        (Photo credit should read WILLIAM WEST/AFP via Getty Images)