Roger Federer could retire in the next "two or three years", says former women's world number one Kim Clijsters.

The Swiss ace won his 20th Grand Slam when he defeated Marin Cilic in five sets at the 2018 Australian Open final on Sunday, 28 January, and further cemented his status as arguably the greatest tennis player of all time.

It continues what has been an unimaginable last 12 months so far as the 36-year-old has defied injuries and age to enjoy a major career renaissance.

Federer began his resurgence by winning the 2017 Australian Open – his first Grand Slam win since 2012. He followed it up with a record-eighth Wimbledon championship and ended the year with seven titles.

The question in the tennis world now is how long Federer will be able to keep up his resurgence.

The world number two has spoken about retirement multiple times, as recently as November last year. However, he stated that he does not want to make the prospect a reality by thinking about it.

"There are many speculations about my retirement, and they won't end until when it gets real," Federer stated. "I try not to think about retirement too often, cause I think the more you think about it, the closer it gets.

"We will see for how much time I will be able to play. My fans know I would like to continue this way, until when I will enjoy playing tennis and am able to reconcile my sports activity with family life."

Clijsters, though, believes the end could be relatively soon as she spoke about Federer's Australian Open win.

"It's very impressive where he has reached now," Clijsters said, as per Tennis World USA. "In two or three years, he could [retire].

"It will be a pity, but in the meantime, it's good to see him with such passion, and when he plays, he does it for pleasure.

"He still gets such results, plays because he loves tennis, [because he wants] to win and he always finds new solutions, he always tries to improve. It's admirable."

As for now, Federer's goal is to ideally continue his current wave of success and eventually replace Rafael Nadal as the new world number one.