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Jack Sock of the United States competes during the 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 competes during the 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)