Mercedes have once again produced an "amazing car" while Ferrari have taken a "step forward", that's according to Felipe Massa.
As the opening pre-season test wrapped up after four days at the Jerez circuit, the Williams driver was asked for his opinion on his team's rivals.
"It's always difficult to give a good answer at the end of the first week of tests," he told ESPN.
"There's still a lot to understand, not just from us but from others as well.
"It's still clear that Mercedes has an amazing car. Ferrari maybe made a step forward but I'm still convinced that we can be there fighting again."
Massa showed solid form during his two days behind the wheel of the Williams FW37.
Wracking up 144 laps, he finished testing with a best time of 1:22.276 and says he is happy with Williams' start to the year.
"I still have a good feeling on the car. At the beginning I think it was very positive because we did not have any reliability problems, which is quite good.
"I think it was a good start and we need to prepare everything we can now to go to Barcelona in a more prepared way. We need to test everything we need thinking about Australia.
"It was a good start and I'm still happy like I was yesterday."