FILE - In this Sept. 11, 2016, file photo, Stan Wawrinka, of Switzerland, returns a shot to Novak Djokovic, of Serbia, during the men's singles final at the U.S. Open tennis tournament, in New York. Defending champion Stan Wawrinka has pulled out of the U.S. Open with an injured knee. Wawrinka announced Friday, Aug. 4, 2017, he would sit out the rest of 2017 because of the knee. (AP Photo/Charles Krupa, File)