Lewis Hamilton is relishing being back at work as Formula One's reigning world champion prepares to defend his crown.

The Mercedes star is this year bidding to win his sixth title, and move to within only one of Michael Schumacher's record haul.

Unlike in recent seasons, however, Hamilton has not arrived in Melbourne for the sport's traditional curtain raiser as the bookmakers' favourite to win the championship.

Indeed, after an impressive eight days of winter testing in Barcelona, it is Hamilton's Ferrari rival, Sebastian Vettel, who will seemingly start the campaign as the driver to beat.

Hamilton, who turned 34 at the beginning of the year, is gearing up for his 13th season in F1 and – following the retirement of Fernando Alonso – is now behind only Kimi Raikkonen in terms of grands prix appearances of the sport's current crop.

Yet, the Englishman, who kept a low profile during the off-season, pointed to his extracurricular activities away from the race track, notably the recent launch of his second Tommy Hilfiger fashion range in Berlin, as a reason why he heads into the forthcoming campaign as motivated as ever.

"I don't struggle with the motivation for that. But, if I didn't have the other things in my life, maybe that would be a different story.

"I am fortunate, however, that I have these other projects. I love going to fashion shows, and being involved in the collection with Tommy Hilfiger. It is awesome."

Hamilton held the upper hand over Vettel last year and in 2017, winning back-to-back championships with two rounds to spare.

Mercedes, meanwhile, are chasing a record sixth driver and constructors' championship double.

But on the eve of this weekend's race, Hamilton's Mercedes boss Toto Wolff warned: "Last year, we had to give it absolutely everything to come out on top, but from what we have learned so far, this year will be even tougher.

"It will push us to our limits – and that's an exciting prospect for every single one of us. We're ready for the fight."

:: Lewis Hamilton was speaking at the launch of the new range of PETRONAS Syntium with CoolTech at PETRONAS' state-of-the-art Global Research & Technology Centre in Turin, Italy.