Valtteri Bottas believes Williams has "improved many of the weaknesses of last year", following Formula 1's first pre-season test.
When the FW38 was launched, technical director Pat Symonds said Williams had focused on the low-speed problems that it struggled with last season.
Early impressions appear positive with Bottas suggesting Williams has made a step forward in that area after his two days in the new car.
"We can feel improvements in areas we attacked," he said. "If it's going to be enough, it's too early to say.
"The car feels better than last year's car, I'm feeling positive, but we still have no idea where we are.
"The main thing is we can see we have improved many of the weaknesses of last year."
The Finn's team-mate Felipe Massa said the initial signs were good, but acknowledged there was still work to do.
"I could feel some improvement but on the first morning I was not happy with the balance. I couldn't make the tyres work but in the afternoon I could.
"Everything I did now is not 100 per cent enough to say [if we succeeded].
"We are working to improve and to have the car ready."
When asked about his thoughts on the improvements made in low-speed by Williams, Symonds said: "Feedback from the drivers and data seem to suggest we have picked up there.
"But now it's down to what the other guys have done."