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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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.
Try-scorer Jenny Davies and fellow prop Megan York reveal to WRU TV's Graeme Gillespie their thoughts after Wales broke their drought in the Women's Rugby World Cup with a 35-3 win over South Africa in Paris.
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Wales' Sam Warburton took a group of employees from Admiral on a behind the scenes tour of the Millennium Stadium and WRU TV tagged along to bring you a snippet of what it's like for the players on match day
WRU TV goes behind the scenes in Paris with adopted reporter Jenny Davies grilling Wales Women's captain Rachel Taylor and uncovers which players she would have on a desert island, among other important things as well.
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