Brian O'Driscoll insists Ireland are braced for a "colossal" contest against France on Friday - with Les Bleus feeling the weight of expectation.
Ireland are reeling from two disappointing victories over Namibia and Georgia and their mood is unlikely to improve with a challenging contest against the rejuvenated hosts.
After losing to Argentina in the tournament opener, France are under immense pressure to overturn the Irish in Paris or else face a humiliating early exit from the tournament.
"It's a fairly big week for both sides. It's enormous," said O'Driscoll.
"There were some big moments in previous World Cups, Lions tours and Six Nations, but they're all behind us.
"So as far as I'm concerned this is the biggest week. I can't say it'll be the biggest week forever but this is a colossal match.
"We're doing everything we can to get the right result.
"I think as players we are confident and there is no reason to panic. There is as much pressure on France as there is on Ireland.
"It is a crunch match. Whoever wins this game will go on to the next phase of the tournament - and both teams know it."
France revived their ambitions of reaching the quarter-finals with a resounding 87-10 triumph over Namibia on Sunday night.
"France were very convincing. They finished well," said O'Driscoll.
"Perhaps they showed that killer instinct that they were lacking in the Namibia match.
"They started slipping off tackles a little bit more and the French are one of the best sides in the world when they're playing with that kind of confidence."
Ireland coach Eddie O'Sullivan admits his side have reached "crisis" point as they enter the decisive period of their World Cup.
"It's a big week that's for sure, although I'm not sure it's my biggest week," said O'Sullivan.
"Once France had lost to Argentina Friday was always going to be a pivotal match.
"The way we have been playing I understand why people are saying that we are in a crisis.
"We know our standard has to go way up. We know if we play poorly we'll get well beaten."