Shanghai: Roger Federer won't find it easy to get back to the top of tennis as he battles to recover from his biggest injury yet, fellow Swiss Stan Wawrinka warned on Wednesday.
Federer, 35, has been out since July for knee rehabilitation after undergoing the first operation of his career on a torn meniscus in February.
The record, 17-time Grand Slam title-winner suffered a freak injury while he was helping bathe his twin daughters after being knocked out of January's Australian Open.
"I think he's practising. I don't know if he's gonna be back in the top four. We will see," said Wawrinka, after beating Kyle Edmund 6-3, 6-4 in the Shanghai Masters' second round.
"It's the first time he has a big injury. It's not easy to come back after that. But if he's fit and 100 percent, for sure he will be really dangerous."
Federer will play the Hopman Cup mixed-teams event in January alongside Wawrinka, his sometime doubles partner, and Belinda Bencic as he makes his return.
The top four is missing both Federer (ranked seventh) and Rafael Nadal (fifth) for the first time since 2003 as signs grow that a new era is evolving.
US Open winner Wawrinka is third in the world behind Novak Djokovic and Andy Murray, but some young new challengers are climbing the rankings.
Wawrinka, who has shaken off the back injury which kept him out of last week's China Open, saw off Britain's Edmund, 21, in just over an hour in Shanghai.
He said he had no problems with his motivation, unlike top-ranked Djokovic who has admitted to burn-out after some adverse results and injuries in recent months.
"The motivation is easy, for sure. I'm really happy with what happened in the US Open," Wawrinka said.
"But there are still a lot of challenges this year with here, with Basel, Paris, London.
"I'm excited to keep playing, especially trying to keep the confidence that I have and trying to push myself the maximum to keep doing some great results."
While Wawrinka motored into the third round, Czech seventh seed Tomas Berdych became the first big casualty when he lost 7-6 (7/4), 7-6 (7/1) to Marcel Granollers.
David Goffin beat Benoit Paire 6-1, 7-6 (7/0) and Gilles Simon ended Chinese hopes when he beat wildcard Wu Di 6-2, 6-2.
Nadal is in action later against Viktor Troicki, before Murray takes on Steve Johnson in the late match on the Shanghai centre court.