PGA Ranking: Roger Federer faces tough task to return to top four: Wawrinka

Roger Federer will not 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 second round of the Shanghai Masters.

"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."

Stan Wawrinka of Switzerland in action against Britain's Kyle Edmund at the Shanghai Masters on October 12, 2016 (AFP)