2014 runner-up Red Bull has emerged as a big question-mark ahead of the new season.
Last year, Daniel Ricciardo was the only driver other than the occupant of a Mercedes to see the view from the top of the podium.
But after the first official test of the 2015 pre-season, the Australian's best aggregate time at Jerez was good for just twelfth overall.
New teammate Daniil Kvyat fared even worse in the camouflaged RB11, outpaced only by the stuttering McLaren-Hondas.
Reliability was also a weakness for Red Bull at Jerez, as even the initially-absent Lotus ultimately managed more laps than the former champions.
Combined, Ricciardo and Kvyat put just 166 laps on the RB11's odometer, compared to the 516 on the tally of the dominant Mercedes.
Speaking to Germany's Auto Motor und Sport, Williams technical boss Pat Symonds agrees that Mercedes is still number 1, with Williams and Ferrari neck-and-neck for second.
The big unknown, Symonds admitted, is Red Bull.
"They have run so little that it is difficult to judge them," he said.
"We (Williams) are where we wanted to be," Symonds added. "Obviously we have no control over what the competition is doing, but it does look as though Mercedes is keeping the handbrake on at the moment.
"And Ferrari has apparently made a good jump," he said.
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