Fernando Alonso, Daniel Ricciardo and Sebastian Vettel have each earmarked Mercedes as the 'reference' following the first day of F1 testing in Jerez.

A year after Mercedes arrived in Jerez with an evident advantage over its rivals under the new regulations, the title-defending team raised eyebrows again as Nico Rosberg completed 157 trouble-free laps in the new W06 - a new record for testing.

Setting the third fastest time, Rosberg's sheer mileage - which amounted to more than two race distances -, certainly caught the eye of his rivals, who all agree that the Mercedes appears to be the early car to beat again.

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"The reference is still Mercedes," said Ferrari's Vettel, who set the overall pace. "They did 150 laps so they have proven they are starting off with a reliable car. Hopefully not as quick as last year - which is what I think everyone is hoping for - but you have to give credit to them because they have done a very good job. I think it will be a surprise if they weren't as strong as they were last year."

Alonso, whose Mercedes-Honda managed just six laps as the team attempts to dial into the all-new power unit, echoes Vettel's sentiments, insisting its pace is expected. Even so, he adds that he doesn't expect the same level of dominance in 2015.

"It is normal. The team that dominated last year, today they are so fast again and doing 150 laps they have an advantage of performance and everything for this year, at least at the start of the season. They have the continuation of a dominant car, so it is probably a bit easier.

"We have a new power unit and we have new streamlined car design, but I don't think Mercedes will win so many races this year. They are the favourites but all the others teams will be better prepared than last year.

Similarly, Ricciardo was impressed with the longevity of Rosberg's track time, but says he is not 'intimidated' by the early signs.

"It is not really a surprise. Their reliability seemed very good, but because they had such a good package last year it is a small refinement on that so they know what they are getting into. The pace looks good and I expect them to be the ones to beat in Melbourne, but I am not intimidated either - I think it is a reachable target for us."