Some sections of the press have lauded the performance of the new Toyota-powered FW30 in winter testing by suggesting that the car could emerge this season as the quickest after likely championship contenders Ferrari and McLaren.
As the car's 2008 tweaked livery was revealed on Monday, Williams was asked if he is cautiously optimistic about improving on last year's solid season.
"Personally, not in the slightest, I don't subscribe to what the press has said," he said.
"Our competitors around us, of which there are many, are just as competitive. To think that we're going to sail into third place in the first few races is pie in the sky.
"If we do, it'll be a tight fit," Williams added.
In a F1 field dominated by works car manufacturers and the lucrative engine drink company Red Bull, Grove based Williams – alongside Force India – is now one of just two true independents.
But Sir Frank insists that this fact does not ease the pain of having failed to win a title since 1997.
"Ten years is a miserably long time, Patrick (Head) and I wince every time we think about it.
"Whether we've turned a corner, we will only be able to tell once the season gets underway," he said.