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Novak Djokovic of Serbia returns a shot during the ATP Cup Group A match against Gael Monfils of France in Brisbane, Australia, on Jan. 6, 2020. (Xinhua/Bai Xuefei)

Novak Djokovic of Serbia returns a shot during the ATP Cup Group A match against Gael Monfils of France in Brisbane, Australia, on Jan. 6, 2020. (Xinhua/Bai Xuefei)