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."
Roger Lewis talks to WRU TV at the announcement of a new £60 million Rugby Services Agreement (RSA) which provides a new and positive contractual relationship between the WRU and its four Regions for the next six years has been agreed.
The Welsh Rugby Union has announced a major new sponsorship deal with Gullivers Sports Travel which will make the UK's leading, and longest established, multi-sport tour operator, an Official Supplier for the rest of the decade.
Having taken part in seven World Cups as a player and coach, Liza Burgess hopes to go out in style this afternoon when Wales take on Australia in the Play-off for seventh and eighth place in the Women's Rugby World Cup in Paris.
WRU Head of Rugby Josh Lewsey met with volunteers across the game earlier this week, championing their role in the sport. He was joined by Wales captain Sam Warburton for part of the day as volunteers took part in a 'Try Out' event for the 2015 Rugby World Cup
Wales Women may not have got the results they wanted recently on the field, but off it they have made massive strides in strength and conditioning - and that is in no small part due to S&C coach Sam Dodge.
Two weeks into the new pitch project at Millennium Stadium, Groundman Craig Campbell takes staff on a unique behind-the-scene tour of the works so far and National Squad Assistant Strength and Conditioning Coach John Ashby and Tour Guide Peter Owen describe what the revamp means for them.