Felipe Massa says he has a "good feeling" with Williams' FW38 despite his running coming to an early end due to a power unit problem.
Massa completed 109 laps on his first day in the car to placed 10th overall, but had to sit in the garage during the final stages of the session due to a mechanical problem.
Massa will remain in the car to complete the first test for Williams on Thursday.
"Today was a good day doing 109 laps, and spending time trying to understand the FW38 for the first time," he said.
"It is always nice to drive the car and understand areas that are better than the old one, but also keep working on understanding what there is to improve in terms of set-up.
"There is definitely a lot to do to get the car where we want it but I had a good feeling, and I am sure we are just at the beginning of many improvements.
"At the very end of the day, unfortunately I had to stop due to an issue in the power unit. We couldn’t finish the programme but we were very close, missing only the final few laps."