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Novak Djokovic of Serbia reacts during the men's singles first round match against Mitchell Krueger of the United States at the Australian Open in Melbourne, Australia, Jan. 15, 2018. (Xinhua/Lui Siu Wai)
Novak Djokovic of Serbia reacts during the men's singles first round match against Mitchell Krueger of the United States at the Australian Open in Melbourne, Australia, Jan. 15, 2018. (Xinhua/Lui Siu Wai)