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Argentina's Juan Martin Del Potro returns the ball to USA's Jack Sock during the final match of ATP Stockholm Open tennis tournament in Stockholm on October 23, 2016. / AFP / JONATHAN NACKSTRAND (Photo credit should read JONATHAN NACKSTRAND/AFP via Getty Images)

Argentina's Juan Martin Del Potro returns the ball to USA's Jack Sock during the final match of ATP Stockholm Open tennis tournament in Stockholm on October 23, 2016. / AFP / JONATHAN NACKSTRAND        (Photo credit should read JONATHAN NACKSTRAND/AFP via Getty Images)