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
Roger Federer of Switzerland serves during the men's singles match against Jack Sock of the United States on day 6 of the Hopman Cup tennis tournament in Perth, Australia, Jan. 4, 2018. (Xinhua/Zhou Dan)
Roger Federer of Switzerland serves during the men's singles match against Jack Sock of the United States on day 6 of the Hopman Cup tennis tournament in Perth, Australia, Jan. 4, 2018. (Xinhua/Zhou Dan)