NEW YORK - SEPTEMBER 7: Stanislas Wawrinka of Switzerland returns ball at US Open semifinal match against Novak Djokovic of Serbia at USTA National Tennis Center on September 7, 2013 in New York Shutterstock Image - NEW YORK - SEPTEMBER 7: Stanislas Wawrinka of Switzerland returns ball at US Open semifinal match against Novak Djokovic of Serbia at USTA National Tennis Center on September 7, 2013 in New York - ID: 156468422