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."
Wales reached their first semi-final since 2013 in the opening round of this season's HSBC World Rugby Sevens Series in Dubai. WRU TV's Graeme Gillespie recaps on what proved to be an eventful tournament
Wales and Ospreys back row James King, who came through the North Wales age grade system, nominated North Wales Regional Community Rugby Manager Marc Roberts, in the WRU's 'Diolch' initiative where squad players thanked those who played a part in their rise to the top.
Beating Merthyr has boosted Carmarthen Quins hopes of reaching a top eight spot in the Principality Premiership. Quins captain Haydn Pugh is delighted with his team's progress and is determined that everyone from the Under 6s through to the senior team, enjoys the ride.
When the Wales squad were asked to nominate individuals who played a big part in their early playing days, Wales and Dragons full back and wing Hallam Amos thought of his former Monmouth School rugby coach John Bevan.
Sam Cross and Luke Treharne have been named as co-captains of the Wales Sevens side as head coach Gareth Williams names his 12-man squad to play in the first leg of the HSBC World Series in Dubai next weekend.
Bedwas head coach Steve Law and players Nicky Griffiths and Alun Rees extol the virtues of the close knit club as they prepare to host third place RGC 1404 - who are one place head of them on the Principality Premiership points table - on Saturday.
Many of the Wales squad have said Diolch (thank you) to someone who played an important part in their early playing days by nominating them to be invited to the Wales v South Africa match. One person, Gowerton Comprehensive geography teacher Mr Dean Mason was nominated by two of his former pupils, Dan Biggar and Sam Davies.
The second WRU disability rugby pilot has been delivered, this time in Caerphilly schools, culminating in a festival at the Caerphilly Centre of Sporting Excellence, Ystrad Mynach tomorrow. Plans are in place to set up a Disability Rugby Club in Newbridge to provide long-term playing opportunities.
WRU TV: Hear from inside the Wales camp as Neil Jenkins discusses this weekend's clash with South Africa and how the squad are looking to put in a strong performance to round of the 2016 Under Armour Series