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Roger Federer (L) of Switzerland and Novak Djokovic of Serbia shake hands during an exihibition match on Kids Tennis Day, held at Melbourne Park in Melbourne, Australia, Jan. 13, 2018. (Xinhua/Bai Xuefei)

Roger Federer (L) of Switzerland and Novak Djokovic of Serbia shake hands during an exihibition match on Kids Tennis Day, held at Melbourne Park in Melbourne, Australia, Jan. 13, 2018. (Xinhua/Bai Xuefei)