Red Bull have admitted they are playing catch-up ahead of Sunday's season-opening Australian Grand Prix, after struggling to come to grips with the new turbo-charged hybrid engine during the pre-season tests.
Finn Hakkinen, who won back-to-back titles in 1998-99 with McLaren, would not discount Sebastian Vettel's chances of winning a fifth consecutive championship for Red Bull but said the constructors' champions were up against it.
"You saw in the testing they weren't able to do enough running," Hakkinen told Reuters.
"They haven't really got a result to the level that they want. In Melbourne, I still think they are very far away.
"They are a probably a couple of months behind the others, so it will take time for them to reach a performance they need."
Mercedes have been the bookmakers' favourites after gaining positive mileage and pace during testing, but suffered an embarrassing setback when Lewis Hamilton's car stopping five minutes into the first free practice session on Friday during his installation lap at Albert Park.
Mercedes said the problem was related to an oil pressure alarm triggering the engine's shutdown but also conceded they had changed the car's power unit in the leadup to the session.
"It definitely was a shock because every lap is so important to collect data from the race track," 45-year-old Hakkinen said of the mishap, which saw Hamilton's car towed off the track and the driver make the long, lonely walk back to the paddock.
"The regulations have changed so dramatically that the teams are having a massive challenge at the moment. There are only a few teams who are really in a stable situation."
Hakkinen, who appeared as a sponsor's ambassador for an anti-drink driving campaign on Friday, said it was too early to tell who would emerge front-runner after the opening round in Melbourne, but was intrigued how the new partnership of Fernando Alonso and Kimi Raikkonen would unfold at Ferrari.
Pundits have speculated the former world champions' rivalry could hurt their team's chances, with neither expected to be content to play second fiddle, but Hakkinen dismissed any idea of tension between the drivers spilling off-track.
"They know the name of the game, so at the end of the day, the results will do all the talking," he said. "But wait a couple of months and we will know."
One of the more exciting prospects to emerge this season was last year's strugglers Williams, Hakkinen said, with the former F1 giants showing signs of revival with new faces in the garage and a new driver in Brazilian Felipe Massa, who crossed over from Ferrari.
"Williams definitely have been developing very positively," Hakkinen said. "I think now we start seeing the real results of the (new) team.
"With Massa being part of the team, it's just strengthened the whole package.
"It will be very interesting to see this week how they will do and I would say they are candidates for certain if they can produce a good result for this weekend."
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