Jerez - Two-time world champion Fernando Alonso believes reigning champions Mercedes will be unable to repeat their domination of the rest of the grid from last season.
Mercedes won 16 of 19 races in 2014 as Lewis Hamilton beat out teammate Nico Rosberg for a second world title.
Alonso has returned to McLaren for the new season after a fruitless five-year spell at Ferrari in search of a third world crown.
There was little for a huge crowd at the first day of pre-season testing in Jerez to cheer as Alonso managed only six laps and finished over 18 seconds behind Sebastian Vettel, who has succeeded the 33-year-old at Ferrari.
Vettel went fastest, but Mercedes impressed once more as Rosberg logged a record 157 laps for the first day of testing.
"Definitely they have an advantage in performance, at least to start the season. They have the continuation of a dominant car, so it is a little bit easier than for us who have a new power unit," said Alonso.
"We have a very streamlined, aggressive car design and I don't think that Mercedes will win so many races this year. They are the favourites, yes, but I think all the other teams are closer than last year."
McLaren have shunned their association with Mercedes in favour a return to Honda engines for the first time in 22 years.
The partnership bore great fruit with four consecutive constructors' championships with the legendary Ayrton Senna at the wheel in the late 1980s and early 90s.
And Alonso preached for patience as he reiterated his belief that the Japanese manufacturer can bring McLaren back to the front of the grid after a couple of barren years.
"It went as expected, it's a slow start. We saw last year how difficult it was for the teams to do laps
"We need time. The car design is quite aggressive, innovative we have things to discover slowly.
"I would like to do more laps for sure because after waiting for two months for the first day of testing six laps is not enough but I think we are all happy, excited and inside the garage we had an amazing atmosphere today.
"I believe in this project. I am so, so happy to be here."
McLaren group CEO Ron Dennis also tried to allay fears over the design of the MP4-30.
"Nothing is overheating, which is normally what happens when you get it wrong on packaging, we've got no burning of heat shields.
"There is every indication that the mechanical design of the car has been well executed."
Jenson Button will get his chance to hopefully put the McLaren through a more prolonged test when he takes the wheel on Monday.