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Roger Federer of Switzerland walks out for the singles round robin match against Jack Sock of the United States during the Nitto ATP World Tour Finals at O2 Arena in London, Britain on Nov. 12, 2017. (Xinhua/Han Yan)
Roger Federer of Switzerland walks out for the singles round robin match against Jack Sock of the United States during the Nitto ATP World Tour Finals at O2 Arena in London, Britain on Nov. 12, 2017. (Xinhua/Han Yan)