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