New York - Novak Djokovic, back from the wilderness with an unlikely Wimbledon triumph, has even better tennis in him, Swiss great Roger Federer says.
"I think he probably still has more left in him. I think he's playing well, but I think he can even play better," Federer said in Flushing Meadows this week as he prepared for Monday's start of the US Open.
Third-seeded Argentine Juan Martin del Potro also backed Djokovic to continue his rise back from a right elbow injury - all the way back to number one in the world.
"I believe in him," Del Potro said.
"I know he has everything to win the first position in the world again.
"But I also know what he has been through to get into this position again. He had frustration with the injury. He leaves early in every tournament. It's not normal for a player like him."
Federer got an up-close look at a rejuvenated Djokovic when the Serb beat him in the final at Cincinnati last week - a triumph that saw Djokovic complete his collection of all nine prestigious Masters 1000 events.
That hardcourt victory stamped Djokovic a US Open favourite as he joins former champions Stan Wawrinka and Andy Murray in returning to the tournament a year after injuries saw them all sit out the last major of the season.
Djokovic pulled the plug on his 2017 season after Wimbledon, although a prolonged break eventually wasn't enough to let him avoid surgery on his elbow.
Wawrinka missed his US title defence last year, part of eight months on the sidelines with a left knee injury, and Murray will be playing his first Grand Slam in more than a year after a hip injury that finally required surgery in January.
Their stories sound similar, but Federer, who at 37 has endured his share of injuries, said there's no magic path back to the top after a significant spell on the sidelines.
"I think it depends on what kind of injury you have had or what kind of process and road you have had to the comeback," he said.
"If you have an injury that always keeps on hurting and you're always playing with fear of your next step or your next match, it can really rob you with your confidence in terms of movement and then you will never be the same player again.
"I think Stan had a little bit of that. So with Novak it has not been the movement. It's been more the arm. We could see that when he came back when he wasn't quite ready yet. His serve wasn't really working. He wasn't hitting his spots. He wasn't serving as well as he was in the past.
"So I don't know if it's a mental thing there or actually the arm doesn't allow you to do it. It really depends on where you come from."
Murray, who hasn't played a Grand Slam since losing in the quarter-finals at Wimbledon last year, agreed.
"All injuries are completely different," he said. "Some are more serious than others. Someone is out for a few weeks with a muscle injury, it's very different to somebody recovering from a surgery on certain parts of the body."
Rather than look to other top players who have returned from injury to guide his comeback, Murray said he's "trying to deal with the situation that I've been in as best as I can myself."
Under no illusions as to his chances of instant Grand Slam success in New York, the three-time major winner is nonetheless certain he can, eventually, return to the top.
"I do feel that once my body is right again I'm sure that my level will be OK to get me competing at the top of the game again," he said.