Alexander Zverev of Germany celebrates after winning the singles group match against Daniil Medvedev of Russia at the ATP World Tour Finals 2019 in London, Britain on Nov. 15, 2019. (Xinhua/Han Yan)

Alexander Zverev of Germany celebrates after winning the singles group match against Daniil Medvedev of Russia at the ATP World Tour Finals 2019 in London, Britain on Nov. 15, 2019. (Xinhua/Han Yan)

Alexander Zverev of Germany 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. 15, 2019. (Xinhua/Han Yan)

Alexander Zverev of Germany celebrates after winning the singles group match against Daniil Medvedev of Russia at the ATP World Tour Finals 2019 in London, Britain on Nov. 15, 2019. (Xinhua/Han Yan)