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Alexander Zverev of Germany returns the ball to Stan Wawrinka of Switzerland during the St. Petersburg Open ATP tennis tournament final match in St. Petersburg, Russia, September 25, 2016. (Photo by Igor Russak/NurPhoto via Getty Images)

Alexander Zverev of Germany returns the ball to Stan Wawrinka of Switzerland during the St. Petersburg Open ATP tennis tournament final match in St. Petersburg, Russia, September 25, 2016. (Photo by Igor Russak/NurPhoto via Getty Images)