The Wales outside-half admits confidence is high among the Red Dragons following back-to-back victories over England and Italy.
But France are unbeaten as well following their success over Scotland and then against England at Twickenham and that has alarm bells ringing for Sweeney.
He said: "I really don't think we have seen the best of this French team yet. You never quite know what you're going to get with France but you know that if they switch on, then they can beat any team in the world. That's the difficulty about preparing to play them.
"They have won two from two yet they have still taken some criticism from their supporters and the media. They are proud rugby people and are bound to react to that."
Sweeney, who won his 19th cap as a replacement against Italy, has been restricted in his time on the pitch by Stephen Jones but insists he is pushing his rival all the way.
He said: "I'm not here to make up the numbers. I'm pushing as hard as I can to get into the starting team but it's hard to force your way in with Stephen and the rest of the boys playing so well at the moment. But that's great news for Wales."
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Grand Slams, Triple Crowns and British and Irish Lions tours - Sir Gareth Edwards did it all. But what are his favourite moments in what was an amazing career? WRU TV finds out - along with his favourite players of today in the third and final instalment of our series with the greatest rugby player ever.
JPR Williams, Gerald Davies and Mervyn Davies were among those in the Welsh ranks when Sir Gareth Edwards began his international career against France nearly 50 years ago, but who was the player who took the young scrum half under their wing?
Wales head coach Rob Howley and captain Alun Wyn Jones are expecting a physical battle against France on Saturday but both are hoping for another impact from the bench as the men in red chase a top-four world ranking.
It's nearly 50 years since the great Sir Gareth Edwards made his debut as a youngster for Wales against France in Stade Colombes. In the first of a series, WRU TV discovers he pocketed a 'special momento' from the game - something he still has today.