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."
The WRU has launched a campaign to create a long term legacy for Welsh club rugby by highlighting the advantages of volunteering. Rhian Edwards, a volunteer at Seven Sisters, has enjoyed many benefits of her volunteering at a grassroots rugby club including being part of the Rugby World Cup volunteer workforce after being nominated by the WRU - and the WRU is asking for more people to develop their 'Welsh rugby roots'.
Wales head coach Warren Gatland and captain Sam Warburton say their first prioirty on Sunday in the opening game of the World Cup is to win against Uruguay and not get overly concerned with points difference.
With only days to go until the biggest rugby competition in the world begins, rugby fans visiting Cardiff this autumn will be able to enjoy a special display at National Museum Cardiff as part of the celebrations surrounding the Rugby World Cup.