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Novak Djokovic (L) of Serbia poses with the World Number one trophy on Day One of the ATP World Tour Finals at the O2 Arena in London, Britain on Nov. 11, 2018. (Xinhua/Han Yan)
Novak Djokovic (L) of Serbia poses with the World Number one trophy on Day One of the ATP World Tour Finals at the O2 Arena in London, Britain on Nov. 11, 2018. (Xinhua/Han Yan)