US's Jack Sock returns the ball to Germany's Dustin Brown during the ATP Stockholm Open tennis tournament in Stockholm on October 20, 2016. / AFP / JONATHAN NACKSTRAND (Photo credit should read JONATHAN NACKSTRAND/AFP via Getty Images)
US's Jack Sock returns the ball to Germany's Dustin Brown during the ATP Stockholm Open tennis tournament in Stockholm on October 20, 2016. / AFP / JONATHAN NACKSTRAND (Photo credit should read JONATHAN NACKSTRAND/AFP via Getty Images)