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LONDON, ENGLAND - NOVEMBER 16: Stan Wawrinka of Switzerland in action against Marin Cilic of Croatia in their Group John McEnroe match on day four of the ATP World Tour Finals at O2 Arena on November 16, 2016 in London, England. (Photo by Ashley Western - CameraSport via Getty Images)

LONDON, ENGLAND - NOVEMBER 16: Stan Wawrinka of Switzerland in action against Marin Cilic of Croatia in their Group John McEnroe match on day four of the ATP World Tour Finals at O2 Arena on November 16, 2016 in London, England. (Photo by Ashley Western - CameraSport via Getty Images)