Sept 2, 2016; New York, NY, USA; Novak Djokovic of Serbia works out on Ashe court under the supervision of coach Boris Becker on day five of the 2016 U.S. Open tennis tournament at USTA Billie Jean King National Tennis Center. Djokovic won when opponent Mikhail Youzhny of Russia retired due to injury. Mandatory Credit: Robert Deutsch-USA TODAY Sports