Englishman Ellis, Les Bleus' defence coach, believes Wales deserve all the plaudits they have received following an outstanding tournament so far.
But he also feels they have yet to be properly tested during a campaign when Wales beat Ireland and Samoa after losing by one point against South Africa.
"Ireland do not score a lot of tries, South Africa do not score a lot of tries," he said.
"They (Wales) have played teams who kick and are defence-orientated, so logically they have not been tested much. But we will test them, without doubt."
France, despite losing to New Zealand and Tonga during a tournament that has seen regular reports surface of rifts between players and coach Marc Lievremont, have reached their fifth successive World Cup semi-final.
Things started to improve in a quarter-final victory over England four days ago, and Ellis claims everything is moving in the right direction.
"France under-performed in the first four games. Since then, they have taken responsibility to make sure they perform right," he added.
"People talk about how much progress the Welsh have made, but I think the French have a hell of a lot more progress to make from how we played in the first four games to how we played against England, and we intend to step up another few steps this coming weekend.
"I think Damien Traille and Imanol Harinordoquy mentioned that they didn't appreciate things coming out in the press about the performances.
"The thing is, Marc says it to the players, he says it to the press quite honestly. I played the game for many years and I didn't like being told that I had had a bad game, either.
"Everything has been taken from that and blown out of proportion. It's good now - it always has been.
"Obviously, when you beat England, which is something the French always like to do anyway, and you make the semi-finals of a World Cup, then things are getting better all the time."
France have beaten Wales six times in the countries' last seven meetings, the latest win being a comprehensive 28-9 Six Nations triumph earlier this year.
But Ellis has no doubt complacency will not creep into the French camp, claiming: "Wales is always a difficult game.
"We've played them in the Six Nations, and when we've won the Six Nations or a Grand Slam the most difficult game has always been Wales.
"The ratings they are getting are justified. They are on the crest of a wave.
"They played very well against South Africa and almost won the game. They've won all the other games, including a very tough one against Samoa, and they beat an in-form Irish team.
"They have some exceptional back-rowers like Sam Warburton, who are fantastic over the top of the ball.
"He is not a big guy, so he is not going to wipe people out who are running at him, but he is very competitive over the top of the ball and we've seen in this tournament that he is winning lots of turnovers, as are (Dan) Lydiate and the other back-rowers."
Ellis is also well aware of the threat Wales present in their back division, notably through centre Jamie Roberts and wing Shane Williams.
"We've played against him (Roberts) on numerous occasions, and he is a major threat. We just have to stop him from doing what he has been doing so far.
"I have memories of Shane Williams when he played for the Ospreys against London Irish in the Heineken Cup last season.
"You control him for the biggest part of the game, then all of a sudden he gets the ball, chips over the top and wins the game for the opposition.
"He is dangerous even on his own try-line, so you just have to be on your mark all the time with him. The idea is you have to make sure he doesn't get the ball by dominating his team and putting them on the back foot."
Ellis' strategic battle with Wales defence specialist Shaun Edwards promises to be one of the semi-final's most alluring sub-plots.
And the Yorkshireman's respect for his Lancastrian rival is considerable.
"We see each other occasionally when I am in England, not socially, but we have a word if our paths cross because he spent a lot of time working with Wasps while I was with Gloucester and London Irish," Ellis said.
"No disrespect to the other defence coaches in the world, but he is the one I have most respect for.
"I say that because he has made himself into an outstanding coach and not just a defensive coach. He's head coach at Wasps, and perhaps with future international teams as well."
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