Stefanos Tsitsipas of Greece returns a shot during the singles semifinal against Roger Federer of Switzerland at the ATP World Tour Finals 2019 in London, Britain on Nov. 16, 2019. (Xinhua/Han Yan)

Stefanos Tsitsipas of Greece celebrates during the singles semifinal against Roger Federer of Switzerland at the ATP World Tour Finals 2019 in London, Britain on Nov. 16, 2019. (Xinhua/Han Yan)

Roger Federer of Switzerland celebrates during the singles semifinal against Stefanos Tsitsipas of Greece at the ATP World Tour Finals 2019 in London, Britain on Nov. 16, 2019. (Xinhua/Han Yan)

Roger Federer of Switzerland returns a shot during the singles semifinal against Stefanos Tsitsipas of Greece at the ATP World Tour Finals 2019 in London, Britain on Nov. 16, 2019. (Xinhua/Han Yan)