“There’s nothing Federer can change in his game to adjust as he did when he came back in 2017,” Rusedski added.
“The older you get, the harder it becomes. He knows the reality of the situation and he realises it’s going to be a hard graft.
“You hate injuries and you hate trying to do a comeback, it’s a real hard thing to do.
“The mental fatigue is so difficult because you still believe there’s something left even if there isn’t.”
The US Open is available to watch on Prime Video from Monday 30th August