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Novak Djokovic of Serbia celebrates after 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/Bai Xuefei)
Novak Djokovic of Serbia celebrates after 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/Bai Xuefei)