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LONDON, ENGLAND - NOVEMBER 17: Stefanos Tsitsipas of Greece celebrates with the trophy after his singles final match victory against Dominic Thiem of Austria during Day Eight of the Nitto ATP World Tour Finals at The O2 Arena on November 17, 2019 in London, England. (Photo by Julian Finney/Getty Images)

LONDON, ENGLAND - NOVEMBER 17: Stefanos Tsitsipas of Greece celebrates with the trophy after his singles final match victory against Dominic Thiem of Austria during Day Eight of the Nitto ATP World Tour Finals at The O2 Arena on November 17, 2019 in London, England. (Photo by Julian Finney/Getty Images)