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Roger Federer of Switzerland returns a shot during the singles group match against Novak Djokovic of Serbia at the ATP World Tour Finals 2019 in London, Britain on Nov. 14, 2019. (Xinhua/Han Yan)

Roger Federer of Switzerland returns a shot during the singles group match against Novak Djokovic of Serbia at the ATP World Tour Finals 2019 in London, Britain on Nov. 14, 2019. (Xinhua/Han Yan)