Felipe Massa has credited Williams' change in fortune during testing to their new engine supplier, saying Mercedes have played a major role.
After two seasons with Renault, the Grove squad signed a long-term deal with the German manufacturer and they immediately showed major signs of improvement during pre-season.
While the Renault-powered teams have struggled due to the rule changes, those using Merc's 1.6-litre turbocharged V6 engines have no reliability problems.
"The best thing Williams did was to sign with Mercedes," Massa is quoted as saying by Autosport. "I'm so happy with the way they are working, with how professional they are and the understanding they have about things.
"They are not just controlling the engine. They understand a lot about the car, a lot about the setup, a lot about so many things that also help us to understand things quickly.
"So it was a very good change."
Having finished ninth in the 2013 Constructors' Championship with just five points, Williams are upbeat of improving this year, especially after the signing of former Ferrari driver Massa.
Asked if the team can maintain the pace of development in 2014, the Brazilian replied: "I'm confident because I think we have good people.
"I really believe in this team and believe that they have the potential to be competitive and that they have the understanding to develop well.
"The correlation between the windtunnel and the track works and that is a good thing. Everything that we brought to the track worked.
"We are just at the beginning. There is still a lot to do and the work from the people [in the team] is on a high level."