The British pair started the first race well but missed a crucial wind shift to end up 11th, as the French boat stormed from 15th to 1st to show what happens when you find the right part of the race track.
In the second race Percy and Simpson were pipped to the line by arch-rivals and Brazilian gold medal favourites Robert Scheidt and Bruno Prada, who also lead the fleet overall after coming fourth in the opener.
"Our position isn’t that important at this stage – it’s more about checking that we are going ok," said Percy, who is seeking his third Olympic gold medal in his fourth Games appearance.
"We obviously missed an important wind shift in the first race but over the week it will hopefully be about having good downwind speed and being steady upwind and that will pay through."
Great Britain's women's match racing team made a steady start to their two-week long Olympic regattam after one win and one loss in their opening two races.
Skipper Lucy Macgregor and her crew, comprising sister Kate and Annie Lush, beat Denmark but lost to Spain, in changeable conditions close to the spectators - an Olympic first.
"It’s a long old event and we’re not used to having just two races in a day so we have to keep our heads screwed on and keep maintaining this energy and we’ll be alright," said Lucy Macgregor.
"It was brilliant out there on the race course – it was an awesome atmosphere – the crowd were really great and obviously we’re not used to having that big a crowd watching us sail.
"It was a very different feeling for us and one I think we coped with really well – so we’re really pleased with that."