Novak Djokovic of Serbia returns a shot during the ATP Cup Group A match against Kevin Anderson of South Africa in Brisbane, Australia, on Jan. 4, 2020.

Novak Djokovic of Serbia celebrates after winning the ATP Cup Group A match against Kevin Anderson of South Africa in Brisbane, Australia, on Jan. 4, 2020.

Novak Djokovic of Serbia returns a shot during the ATP Cup Group A match against Kevin Anderson of South Africa in Brisbane, Australia, on Jan. 4, 2020.

Novak Djokovic of Serbia returns a shot during the ATP Cup Group A match against Kevin Anderson of South Africa in Brisbane, Australia, on Jan. 4, 2020.

Novak Djokovic of Serbia serves during the ATP Cup Group A match against Kevin Anderson of South Africa in Brisbane, Australia, on Jan. 4, 2020.

Kevin Anderson of South Africa returns a shot during the ATP Cup Group A match against Novak Djokovic of Serbia in Brisbane, Australia, on Jan. 4, 2020. (Photos: Xinhua)