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Dominic Thiem of Austria returns a shot during the singles group match against Roger Federer of Switzerland at the ATP World Tour Finals 2019 in London, Britain on Nov. 10, 2019. (Xinhua/Han Yan)

Dominic Thiem of Austria returns a shot during the singles group match against Roger Federer of Switzerland at the ATP World Tour Finals 2019 in London, Britain on Nov. 10, 2019. (Xinhua/Han Yan)