"We didn’t have a single issue with the FW37, which is impressive for the first day with a new car," he said.
"The car was consistent, so it’s a really good start to the week and we are pleased that it’s still a step up from the car we had at the end of last season.
"There are a lot of strengths in this car and we’ve made good progress in just one day, so I am very pleased."
Mercedes-powered Williams are targeting wins this season after returning to the top three and finishing ahead of Ferrari in 2014.
Bottas expected the car to improve "massively" over the course of testing and the season, which starts in Australia on March 15.
"The car was already good last year but it feels better already," he told reporters.
(Reporting by Alan Baldwin, editing by Justin Palmer)
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