Although Valtteri Bottas is "confident" Williams will continue improving, he has some reservations as to whether they can catch Mercedes and Ferrari.
The Grove team ended last year's Championship as Mercedes' closest challenger but have fallen by Ferrari in this year's opening races.
While Williams have been comfortably inside the points, it is Ferrari should have joined Mercedes in the battle for the top three spots.
And it's a gap that Bottas isn't certain Williams can close.
"We need to make sure the new parts we"ll have for Barcelona will really work as expected, because if that's the case we will be closer to Mercedes and Ferrari in Spain," he said in a blog on his team's official website.
"I'm confident that will be the case because since last year we have had a great development program but this year it will be difficult to completely close the gap to the top two teams because Ferrari made a huge step forward since the first race.
"I'm confident that we will keep improving, but of course it will then depend on how much the teams in front improve as to whether we can catch up with them or not."
The Finn, though, does at least go into this three-week on a high note after a good night in Bahrain had him holding off Sebastian Vettel to finish fourth.
"I have to say it's great to have another race weekend immediately after a difficult one, like the Chinese Grand Prix was for us.
"And even better when we could see we made progress after the race in Shanghai, so it's now nicer to go to the break before the European races knowing we're making progress.
"As for myself, I had a good weekend, a very good lap in qualifying and a great race, so it's better to have a longer break on the back of such a result."