LONDON, ENGLAND - NOVEMBER 06: Juan Martin del Potro of Argentina meets the 2013 Barclays Ball Kids and takes part in some training exercises during day three of the Barclays ATP World Tour Finals at O2 Arena on November 6, 2013 in London, England. (Photo by Clive Brunskill/Getty Images for Barclays)
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Getty Images Sport Clive Brunskill