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PERTH, AUSTRALIA - JANUARY 04: Roger Federer of Switzerland serves to Alexander Zverev of Germany during the men's singles match on day four of the 2017 Hopman Cup at Perth Arena on January 4, 2017 in Perth, Australia. (Photo by Paul Kane/Getty Images)

PERTH, AUSTRALIA - JANUARY 04:  Roger Federer of Switzerland serves to Alexander Zverev of Germany during the men's singles match on day four of the 2017 Hopman Cup at Perth Arena on January 4, 2017 in Perth, Australia.  (Photo by Paul Kane/Getty Images)