LONDON, ENGLAND - NOVEMBER 14: Jack Sock of The United States is congratulated on victory by Marin Cilic of Croatia following their singles match on day three of the Nitto ATP World Tour Finals at O2 Arena on November 14, 2017 in London, England. (Photo by Clive Brunskill/Getty Images)
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LONDON, ENGLAND - NOVEMBER 14: Jack Sock of The United States signs a camera following victory during the singles match against Marin Cilic of Croatia on day three of the Nitto ATP World Tour Finals at O2 Arena on November 14, 2017 in London, England. (Photo by Clive Brunskill/Getty Images)
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Getty Images Sport Clive Brunskill

LONDON, ENGLAND - NOVEMBER 14: Jack Sock of The United States makes his way out onto court during the singles match against Marin Cilic of Croatia on day three of the Nitto ATP World Tour Finals at O2 Arena on November 14, 2017 in London, England. (Photo by Clive Brunskill/Getty Images)
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Getty Images Sport Clive Brunskill

LONDON, ENGLAND - NOVEMBER 14: Marin Cilic of Croatia celebrates winning the 1st set during the singles match against Jack Sock of The United States on day three of the Nitto ATP World Tour Finals at O2 Arena on November 14, 2017 in London, England. (Photo by Clive Brunskill/Getty Images)
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Getty Images Sport Clive Brunskill

LONDON, ENGLAND - NOVEMBER 14: Jack Sock of The United States celebrates during the singles match against Marin Cilic of Croatia on day three of the Nitto ATP World Tour Finals at O2 Arena on November 14, 2017 in London, England. (Photo by Clive Brunskill/Getty Images)
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Getty Images Sport Clive Brunskill