The double podium for Felipe Massa and Valtteri Bottas in the Abu Dhabi Grand Prix was enough for Williams to finish ahead of Ferrari this season for the first time since it won the championship in 1997.
But despite its impressive turnaround, Williams says it must push to do even better in 2015 as it bids to launch a full-on challenge for wins and the championship.
Deputy team principal Claire Williams said: "We are pleased with P3 but we have work to do.
"Frank is saying, 'why is everyone celebrating P3? We are here to win!'
"So we have to make sure we continue with work over the winter and move forward.
"Hopefully we can close that gap to Mercedes, and hope our competitors don't close the gap as much as we do, so we can take the fight to Mercedes next year."
After its abysmal 2013 campaign, where it ended up ninth overall, a switch to Mercedes power paid dividends for Williams this year.
But the team is insistent that its success this year was not just because of its engine change.
Williams added: "I think everyone has seen the transformation the team has made going from P9 to P3.
"It is an impressive turnaround in anyone's book, and something we are proud of. It has taken a lot of hard work, a lot of diligence has gone into it, and it hasn't just been a fluke.
"One important thing has been sustained success, because it is not just one or two races where we have a good haul.
"There has been consistency there, which we know we need to continually achieve if we want to achieve more."
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