Felipe Massa has warned against writing off Red Bull, adamant that once they sort out the reliability, the RB10 will be on the pace.
Red Bull have reigned supreme in recent years, winning four successive Drivers' and Constructors' Championships.
However, if pre-season testing is anything to go by the team's dominance is under threat.
"We know that they have a good car and we know that they know how to make a winning car..."
The team's longest single run out on track has been a meager 20-lap stint as reliability issues with both the RB10 and its Renault engine have dogged the team.
Daniel Ricciardo, though, did manage 66 laps in total in Bahrain on Friday with a best time of 1:35.743, which put him third on the day's timesheets, 0.173s off the pace.
As such Massa has warned against discounting the Bulls.
He told Autosport: "We saw that the Red Bull struggled every day, but the lap times they did [on Friday] were not so bad.
"You can't put Red Bull and Lotus on the side [as contenders] because when they fix the problems, we know that they have a good car and we know that they know how to make a winning car.
"It is always nice to say that a Red Bull is struggling and that maybe it can be fighting with the Caterhams, but forget it. It's not like that.
"They have a problem to solve, but when it is solved they will be there."
As for his own team and their FW36, the Williams drier completed 103 laps on Friday with a best time of 1:36.507.
"There is still a lot to do and a lot to understand and I hope we can finish the day with more information about everything.
"[Friday] was actually my first day because I just did one afternoon last week and we know how important it is to drive, to understand.
"I'm happy with the way we are working, but still we need to understand the right direction, the right set-up, the right car to start in Australia.
"I hope we can have more working on the set-up to understand and be a little bit more prepared for Australia."