MELBOURNE, Australia - Lewis Hamilton is shaping as a wildcard challenger to Sebastian Vettel as the German embarks on a bid for an historic fourth consecutive Formula 1 title at this weekend's Australian GP.Hamilton, 2008 World champion, and new Mercedes team mate Nico Rosberg were the best performers in off-season testing, giving the German brand hope as it bids to gatecrash the F1 cartel of Red Bull, Ferrari and McLaren.The ultra-fast Briton, who quit his long-term employer McLaren for Mercedes in 2012, is attempting to succeed where seven-times champion Michael Schumacher failed in leading the Silver Arrows back to success.ENTERING THE UNKNOWNFerrari's Fernando Alonso, who missed a third title by three points in 2012, and McLaren's 2009 champion Button who finished a disappointing fifth in 2012, are Vettel's other main challengers.Hamilton said: "The teams don't know how they compare in performance and with big changes to the tyres again for 2013 we just don't know what will happen in the race. We had a good, reliable car during testing so that's a positive we can take with us into this weekend but we know that, in terms of performance, everything begins again from zero in opening practice."NO MORE GAMESAfter the intrigue of testing, the game of cat-and-mouse will be over when the cars take to the Albert Park street circuit for Friday's (March 15) opening practice for the first of three Asia-Pacific races ahead of Malaysia and China. Red Bull's Vettel, 25, wants to become the youngest driver to win four World titles in a row, a feat only achieved by F1 legends Juan Manuel Fangio and Schumacher.The young German was only eighth-fastest in testing at Barcelona this month but despite the inconclusive session and the performance of the new Pirelli tyres, he was upbeat about the 2013 season. "We learned a lot through the tests and we'll be bringing that new knowledge with us to Australia. I am already really looking forward to it,."HOME GROUNDAustralian team mate Webber, who will be competing in his 12th home GP, agreed it was difficult to judge which team had the quickest car ahead of the opening race."It's difficult to read, and in general we do have some quick teams, there's no question about it," he said. "Mercedes can certainly pull a single lap out that's pretty strong. Ferrari will definitely be there and they'll be challenging for victories, as will Red Bull, as will Lotus, as will McLaren, but as the season goes on I think that will close down a little bit and the teams that are more organised and develop well will be stronger."FIERCE CONTENDERAlonso brilliantly mastered performance problems with his Ferrari to take last year's title down to the last race, after leading by 34 points at the halfway point of the season.The Spaniard has a consistent record in Melbourne, finishing in the top five placings in the last nine years.McLaren, which has won the Australian GP 11 times, will be pinning their race hopes on Britain's Button, a three-time winner in Melbourne, who is now partnered by former Sauber ace Sergio Perez."I don't think any team really knows or understands the competitive order for 2013," said Button. UNPREDICTABLE"It's been an extremely hard-to-read winter: varying fuel-loads and levels of tyre degradation mean that it's hard to accurately predict who'll arrive in Australia with the best-sorted car. But that's part of the game."Five new drivers will take their places in the line-up, with Esteban Gutierrez replacing fellow Mexican Perez at Sauber and Valtteri Bottas of Finland representing Williams.Dutch driver Giedo van der Garde joins Malaysian-owned Caterham and Max Chilton, son of a British tycoon, and France's Jules Bianchi form an all-new partnership at Marussia.But the starting grid has been cut to 11 from 12 teams after Spanish strugglers HRT fell victim to financial problems. Stay with Wheels24 for the 2013 Formula 1 season – fresh reports every day.