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Roger Federer of Switzerland walks out on court for his third and final round robin match against Marin Cilic of Croatia during the Nitto ATP World Tour Finals at O2 Arena on November 16, 2017 in London, England. (Photo by Alberto Pezzali/NurPhoto via Getty Images)

Roger Federer of Switzerland walks out on court for his third and final round robin match against Marin Cilic of Croatia during the Nitto ATP World Tour Finals at O2 Arena on November 16, 2017 in London, England.  (Photo by Alberto Pezzali/NurPhoto via Getty Images)