"This is his most difficult mission" - Roger Federer's biographer on why his 2021 comeback will be tougher than 2017

20-time Major winner Roger Federer's comeback next year is eagerly anticipated by almost the entire tennis community. Federer has been away from the court since February due to a double knee arthroscopic surgery, and he is expected to return in early 2021.

While Federer has been out of commission, Rafael Nadal has tied his record Slam tally and Novak Djokovic has got to within touching distance of breaking his most weeks at No. 1 record. Given the circumstances, Federer will be under immense pressure to get his 2021 comeback off the blocks quickly.

Many of Federer's fans believe the Swiss can put together another fairytale season in 2021, the way he did in 2017 after a similar injury layoff. But German writer Simon Graf, who recently wrote a biography on Roger Federer titled "Roger Federer: Phenomenon. Enthusiast. Philantrhopist.", thinks otherwise.

"This is Roger Federer's most difficult mission," Graf was reported as saying. "You're used to Roger Federer's miracles and fondly remember 2017. But this time, getting back to work will be far more complicated."

While making a return to the top of the game will certainly be a tough ask for Roger Federer, this is not exactly uncharted territory for him. In 2017, Federer proved everyone wrong as he overcame a knee surgery to win the Australian Open and Wimbledon.

However, there is a big difference between the Swiss Maestro's comeback in 2017 and his expected comeback next year. Roger Federer's knee injury in 2020 was more serious than his injury four years ago, plus he is now at a much more vulnerable age than before.

Federer's 2017 run in Melbourne was the stuff of legend. He came into the tournament ranked No. 17 in the world, but took down five top 10 players - including Rafael Nadal - en route his 18th Major title.

But his fans would do well to keep their expectations low this time around, as suggested by Graf.

Roger Federer, who will turn 40 next year, is expected to be taking part in the 2021 Australian Open, which has reportedly been postponed to early February.

Roger Federer recently increased his intensity, now he's testing his body more: Simon Graf

Roger Federer celebrates winning the 2017 Australian Open

Roger Federer has returned to training under his coach Severin Luthi. But there has been no account of Federer's progress on social media, and it remains to be seen whether the Swiss Maestro is at the right physical level to play competitive tennis when he returns to the court.

According to Graf, Roger Federer and his team haven't roped in any professional players for training. The Swiss apparently has decided to keep his practice sessions light, hitting mostly with former ATP player Ivo Heuberger since mid-August.

The German writer also added that while Federer has upped the intensity of his training regime, there is a lot of work left to do before he can start competing for Slams or even titles.

"Federer recently increased the intensity - now he's testing his body more," Graf reportedly added. "But until the form that would allow him to be competitive in Grand Slam tournaments, even to play for the title, there is still a lot to be done."

Whether Roger Federer will be able to challenge for the Slams next year is very much up in the air right now. But fans will still be happy to know that the Swiss legend is on track to play at the Australian Open.