Dominic Thiem (R) of Austria greets Novak Djokovic of Serbia after their the singles group semifinal at the ATP World Tour Finals 2020 in London, Britain, Nov. 21, 2020. (Xinhua/Han Yan)

Novak Djokovic of Serbia hits a return during the singles group semifinal against Dominic Thiem of Austria at the ATP World Tour Finals 2020 in London, Britain, Nov. 21, 2020. (Xinhua/Han Yan)

Novak Djokovic of Serbia enters the court before the singles group semifinal against Dominic Thiem of Austria at the ATP World Tour Finals 2020 in London, Britain, Nov. 21, 2020. (Xinhua/Han Yan)

Dominic Thiem of Austria celebrates after winning the the singles group semifinal against Novak Djokovic of Serbia at the ATP World Tour Finals 2020 in London, Britain, Nov. 21, 2020. (Xinhua/Han Yan)

Novak Djokovic of Serbia hits a return during the singles group semifinal against Dominic Thiem of Austria at the ATP World Tour Finals 2020 in London, Britain, Nov. 21, 2020. (Xinhua/Han Yan)

Dominic Thiem of Austria celebrates after winning the the singles group semifinal against Novak Djokovic of Serbia at the ATP World Tour Finals 2020 in London, Britain, Nov. 21, 2020. (Xinhua/Han Yan)

Dominic Thiem of Austria celebrates during the singles group semifinal against Novak Djokovic of Serbia at the ATP World Tour Finals 2020 in London, Britain, Nov. 21, 2020. (Xinhua/Han Yan)

Dominic Thiem (R) of Austria greets Novak Djokovic of Serbia after their the singles group semifinal at the ATP World Tour Finals 2020 in London, Britain, Nov. 21, 2020. (Xinhua/Han Yan)

Novak Djokovic of Serbia reacts during the singles group semifinal against Dominic Thiem of Austria at the ATP World Tour Finals 2020 in London, Britain, Nov. 21, 2020. (Xinhua/Han Yan)

Dominic Thiem of Austria celebrates during the singles group semifinal against Novak Djokovic of Serbia at the ATP World Tour Finals 2020 in London, Britain, Nov. 21, 2020. (Xinhua/Han Yan)

Novak Djokovic of Serbia hits a return during the singles group semifinal against Dominic Thiem of Austria at the ATP World Tour Finals 2020 in London, Britain, Nov. 21, 2020. (Xinhua/Han Yan)

Dominic Thiem of Austria hits a return during the singles group semifinal against Novak Djokovic of Serbia at the ATP World Tour Finals 2020 in London, Britain, Nov. 21, 2020. (Xinhua/Han Yan)

Dominic Thiem of Austria celebrates after winning the the singles group semifinal against Novak Djokovic of Serbia at the ATP World Tour Finals 2020 in London, Britain, Nov. 21, 2020. (Xinhua/Han Yan)

Novak Djokovic of Serbia reacts during the singles group semifinal against Dominic Thiem of Austria at the ATP World Tour Finals 2020 in London, Britain, Nov. 21, 2020. (Xinhua/Han Yan)

Dominic Thiem of Austria celebrates during the singles group semifinal against Novak Djokovic of Serbia at the ATP World Tour Finals 2020 in London, Britain, Nov. 21, 2020. (Xinhua/Han Yan)

Dominic Thiem of Austria hits a return during the singles group semifinal against Novak Djokovic of Serbia at the ATP World Tour Finals 2020 in London, Britain, Nov. 21, 2020. (Xinhua/Han Yan)

Novak Djokovic of Serbia reacts during the singles group semifinal against Dominic Thiem of Austria at the ATP World Tour Finals 2020 in London, Britain, Nov. 21, 2020. (Xinhua/Han Yan)

Dominic Thiem of Austria celebrates after winning the the singles group semifinal against Novak Djokovic of Serbia at the ATP World Tour Finals 2020 in London, Britain, Nov. 21, 2020. (Xinhua/Han Yan)

Novak Djokovic of Serbia reacts during the singles group semifinal against Dominic Thiem of Austria at the ATP World Tour Finals 2020 in London, Britain, Nov. 21, 2020. (Xinhua/Han Yan)

Novak Djokovic of Serbia hits a return during the singles group semifinal against Dominic Thiem of Austria at the ATP World Tour Finals 2020 in London, Britain, Nov. 21, 2020. (Xinhua/Han Yan)

Dominic Thiem of Austria hits a return during the singles group semifinal against Novak Djokovic of Serbia at the ATP World Tour Finals 2020 in London, Britain, Nov. 21, 2020. (Xinhua/Han Yan)

Dominic Thiem of Austria hits a return during the singles group semifinal against Novak Djokovic of Serbia at the ATP World Tour Finals 2020 in London, Britain, Nov. 21, 2020. (Xinhua/Han Yan)

Dominic Thiem of Austria celebrates after winning the the singles group semifinal against Novak Djokovic of Serbia at the ATP World Tour Finals 2020 in London, Britain, Nov. 21, 2020. (Xinhua/Han Yan)

Novak Djokovic of Serbia hits a return during the singles group semifinal against Dominic Thiem of Austria at the ATP World Tour Finals 2020 in London, Britain, Nov. 21, 2020. (Xinhua/Han Yan)

Novak Djokovic of Serbia reacts during the singles group semifinal against Dominic Thiem of Austria at the ATP World Tour Finals 2020 in London, Britain, Nov. 21, 2020. (Xinhua/Han Yan)