"The mental side was not that easy as restore the arm was," Dominic Thiem said.
The former champion has been dealing with a stress fracture in her back.
The 2014 US Open finalist has been snake-bitten by injuries.
The Spaniard suffered a spinal injury this spring and has been unable to recover.
US Open will celebrate Billie Jean King and 50th anniversary of equal prize money on Monday night.
Novak Djokovic tuned up for the US Open playing tennis in Times Square.
Jack Sock and John Isner receive wild card into men's doubles field.
