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