Felipe Nasr said he was very happy after driving a Formula 1 car for the first time.
The Brazilian driver has joined Williams as the team's official test and reserve driver.
Nasr took to the track on Saturday and completed 87 laps on his way to setting the fourth fastest time.
"I am very happy to have driven a Formula 1 car for the first time, and reaching 87 laps made it also a very productive day," he said.
"There were so many things going through my head before sitting in the car."
"It was what I was expecting though: very different to any other car I have ever driven with the amount of downforce, brake efficiency and the power. It was a learning process for me, but I was giving feedback to the engineers all day and I think we found some good things."
"I'm so pleased to have had this opportunity and I am now looking forward to the next time."
Chief test and support engineer Rod Nelson said he was pleased with Nasr's feedback.
"He did a very good job for us. We were keen for him to get some experience of the FW36 under his belt as it's important that he gets up to speed quickly so he can help us with the development of the car."
"He had a very sensible measured approach so we are happy with what we have seen so far. He gives good feedback, works well with his engineers and is also quick, and so we couldn't ask for more and are also looking forward to running him at some races this season."