"We got the win and the four tries we need against Japan, but we made it difficult for ourselves. We didn't underestimate them and they were strong in defence and positive in attack," said Warburton.
"We know France are going to be very strong - it is the crunch game for us. We need to have the biggest game of our lives."
Wales Coach Pat Horgan mirrored those sentiments after watching his side beat Japan 33-10 to make it two wins from two outings.
"We are a bit disappointed because there were too many errors against Japan. But we've picked up two bonus points to take into our final Pool match on Saturday and that is pleasing," said Horgan.
"The other good thing is that we have 26 fully fit players to pick from this weekend. Saturday is going to be the biggest occasion of their rugby lives so far for this group.
"Our win over France in the Six Nations championship will count for nothing on Saturday and I'm sure the French will be happy if this weather continues until then."