What a difference a year can make. Twelve months ago the tennis world wondered what Roger Federer's body and mental state would be in after a lengthy absence. Two Grand Slams later and seven titles earning $18m in prize money to bolster his bulging bank balance it leads us to one question: How had he been written off as a spent force?

As the ageless superstar heads into a new season as the favourite for a sixth Australian Open title and an unprecedented 20th Grand Slam, there could be no stopping the 36-year-old and here's why.

It's all about survival of the fittest.

Of the so-called Big Five in the men's game, Federer is the run-away healthiest member of the elite gang.

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Briton Andy Murray's gut-wrenching decision to under go hip surgery has taken him out of the mix for five months at least but the other three players aren't exactly in fine working order.

Serb Novak Djokovic is still dealing with pain from his elbow complaint which kept him sidelined since Wimbledon. Stan Wawrinka is showing match rustiness after spending six months on the treatment and physio table with a knee issue. World No 1 Rafael Nadal, meanwhile, couldn't complete a whole season without his troublesome knee injury flaring up again.

Blisters have also been a problem for Nadal during his practice sessions and the scorching temperatures in Melbourne will not help his situation.

Nadal was Federer's biggest rival last year but even the Spaniard was flummoxed as to how tackle the rejuvenated Swiss wonder.

In the four matches they played last year, Nadal won just two sets. Both came in Melbourne in their first Grand Slam final head-to-head since 2011.

Competition away from Big Five remains weak.

For all the talk of the next generation coming through in the men's game, still no man born after 1988 has yet claimed a major.

Defending men's singles champion Switzerland's Roger Federer juggles the ball as he prepares to serve during a practice session ahead of the Australian Open tennis championships. Photo / AP
Defending men's singles champion Switzerland's Roger Federer juggles the ball as he prepares to serve during a practice session ahead of the Australian Open tennis championships. Photo / AP

Milos Raonic, now 27, climbed to No 3 in the world rankings in November 2016 yet has reached one Grand Slam final in 26 attempts and reached two other semi-finals. Grigor Dimitrov is another player of unfulfilled talent. The Bulgarian is a year younger than Raonic and while he has an all-round asethically pleasing style, hasn't flexed his muscles in the majors. A return of two semifinals and one quarter-final showing is remarkably bleak for a player who promised so much.

The moment may have passed for Raonic, Dimitrov and even injury-struck Kei Nishikori but the new guard have time on their hands.

Alexander Zverev couldn't handle the pressure at Flushing Meadows as one of the pre-tournament favourites and fourth seed and is yet to move past the last 16 of a slam.

Russia's Andrey Rublev and Canada's Denis Shapovalov are exceptional explosive talents but at 20 and 18-years-old are still a season or two away from troubling and unseating Federer from his throne.

Marin Cilic was the last man other than a Big Five member to win a major in New York in 2014. But his emotional loss to Federer in last year's Wimbledon final is likely to have left mental scars.

The most obvious player to challenge Federer's potential stranglehold could yet be Aussie Nick Kyrgios who has shown he is not intimidated to get in the Swiss master's face.

Kyrgios arguably provided Federer with one of his toughest matches last year in Miami and only lost on a third-set tie-break.

The mercurial Australian has started the season well with victory in Brisbane earning unlikely fans along the way.

A colourful character, the pressure of ending Australia's long search for their next men's slam champion could yet prove a task beyond his broad and bold shoulders this year at least.

Federer is a liberated force.

Defending men's singles champion Switzerland's Roger Federer poses for a photo with his trophy during a ceremony for the official draw at the Australian Open tennis championships. Photo / AP
Defending men's singles champion Switzerland's Roger Federer poses for a photo with his trophy during a ceremony for the official draw at the Australian Open tennis championships. Photo / AP

From the moment the Swiss rocked up in Australia, cosying up to take a selfie with the "world's happiest animal", his demeanour has been one of contentment.

While Federer has previously revealed he is a "little scared of dogs", he was perfectly on board with allowing a handful of quokkas at Rottnest Island, off Perth, clamber over him while he lined up the perfect photo that once posted on his social channels went viral.

It was all a PR ploy to promote his appearance at the Hopman Cup where Federer's liberated and carefree nature saw him win a second title in Perth, 17 years after his first success alongside Martina Hingis.

Federer has nothing left to prove in the game. He could swan through the season in exhibition mode confident in how to get the best from his ageing but seemingly ageless body.

His love of playing the game has never diminished.

Even in those barren years Federer's energy and passion for tennis remained.

His decision to cherry-pick his events and not chase ranking points has safeguarded his body and sustained the hunger.

"Federer is a tennis maniac," Rogers Cup tournament director Eugene Lapierre said at the end of last season.

"His agent Tony Godsick told me that recently, before one of his matches, he sent messages of support to the Swiss team 14 years and under who participated in an international competition! He follows everything, absolutely everything."

That passion was also evident in his invitation to up and coming Canadian player Felix Auger-Aliassime to join him in a training session in Dubai last month.

Auger-Aliassime, who won the US Open boys' title in 2016, became the latest name in a growing list of players who have had the honour of joining Federer in the off season.

Australia's Thanasi Kokkinakis and American Jared Donaldson are two of the young guns who have been invited to train with Federer in recent times.

Federer wants to inspire the next generation to take over his mantle. But as the signs indicate, it could be some time before they are able to remove him from his perch.