Roger Federer believes he will know by "April-May" whether or not he will be able to return to tennis.

The 40-year-old, winner of 20 Grand Slam titles, has not played since a Wimbledon quarterfinal exit at the hands of Poland's Hubert Hurkacz in July last year before having to undergo knee surgery.

"I want to come back strong and give it everything that I have," the Swiss star said Wednesday.

"To give you a little bit of an update. I have a very interesting and important next few months ahead of me. I think I will know a lot more by April-May.

Australian Open: Rafa roars back, breaks record  

"The drive is still there. I am motivated to do my work."

Federer played just 13 matches in 2021. He also underwent two knee surgeries in 2020, when he played only six times.

His absence has seen the former world number one slip to 30 in the rankings.

"I'm back in the gym again tomorrow. I'm working as hard as I'm allowed to, so it's still good times even though it's a little bit slow," Federer told one of his sponsors, Credit Suisse, in a video conference.

Nadal feels lucky to be part of dominant 'Big Three'  

"I would love to do way more, but the doctors and everybody's holding me back a little bit."

Federer also hailed his friend and rival Rafael Nadal, who captured a record-setting 21st Grand Slam title at the Australian Open last weekend.

"It was nice to see his emotions after such a hard-fought victory," he said of the 35-year-old Spaniard who has battled his own injury problems over the years.

"Rafa told me that he hadn't been feeling well with his body -- now he's holding up the Australian Open trophy. He's a great example, a great role model."

Federer's 20th and most recent Grand Slam title came at the 2018 Australian Open.