Daniil Medvedev of Russia returns a shot during the singles group match against Rafael Nadal of Spain at the ATP World Tour Finals 2019 in London, Britain on Nov. 13, 2019. (Xinhua/Han Yan)

Rafael Nadal of Spain serves during the singles group match against Daniil Medvedev of Russia at the ATP World Tour Finals 2019 in London, Britain on Nov. 13, 2019. (Xinhua/Han Yan)

Rafael Nadal of Spain walks into the court before the singles group match against Daniil Medvedev of Russia at the ATP World Tour Finals 2019 in London, Britain on Nov. 13, 2019. (Xinhua/Han Yan)

Daniil Medvedev of Russia competes during the singles group match against Rafael Nadal of Spain at the ATP World Tour Finals 2019 in London, Britain on Nov. 13, 2019. (Xinhua/Han Yan)

Rafael Nadal of Spain returns a shot during the singles group match against Daniil Medvedev of Russia at the ATP World Tour Finals 2019 in London, Britain on Nov. 13, 2019. (Xinhua/Han Yan)

Daniil Medvedev of Russia reacts during the singles group match against Rafael Nadal of Spain at the ATP World Tour Finals 2019 in London, Britain on Nov. 13, 2019. (Xinhua/Han Yan)

Rafael Nadal of Spain celebrates during the singles group match against Daniil Medvedev of Russia at the ATP World Tour Finals 2019 in London, Britain on Nov. 13, 2019. (Xinhua/Han Yan)

Stefanos Tsitsipas of Greece celebrates after winning the singles group match against Alexander Zverev of Germany at the ATP World Tour Finals 2019 in London, Britain on Nov. 13, 2019. (Xinhua/Han Yan)

Stefanos Tsitsipas of Greece celebrates during the singles group match against Alexander Zverev of Germany at the ATP World Tour Finals 2019 in London, Britain on Nov. 13, 2019. (Xinhua/Han Yan)

Stefanos Tsitsipas of Greece returns a shot during the singles group match against Alexander Zverev of Germany at the ATP World Tour Finals 2019 in London, Britain on Nov. 13, 2019. (Xinhua/Han Yan)