Jerez F1 testing: New Williams a step forward - Valtteri Bottas

Although a delay with the arrival of final components meant the FW37 took to the track later than planned on Sunday, Bottas was able to complete 73 laps and he was very encouraged with how things went.

"We had some delays yesterday. Some parts were a bit late and everything was a little bit on the limit to get ready for the first day," he explained.

"But after we got running, everything went really well.

"The initial feeling was good. I feel it's a good base to build on."

Asked if he already felt the car was a step forward over the 2014 machine that took Williams to third in the championship, Bottas replied: "It is, yes. I know that.

"And this is only the first day and we will improve massively during the testing and during the season as well.

"The car was behaving really consistently. I could do really consistent times on the long runs.

"Immediately after the first few laps I had a good feel of the car.

"We were not really trying to set any laptimes. We were just going through the programme and experiencing the new car, and that went really well."

The Finn highlighted aerodynamics as a clear improvement.

"They're a bit more consistent through the corner," he said.

"So that was a nice thing to feel, because you get more trust in the car.

"The car was already good last year, but it feels better already.

"We still have lots to do to unlock more performance, but we don't need to do it here, we still have plenty of time."

Bottas's tally of 73 laps was still 84 behind the amount Nico Rosberg achieved in the Mercedes, but the Williams driver shrugged off the comparison.

"We never planned 150 laps anyway because we had some changes to make which took some time," he said. "Maybe they had a different target."