The New Zealand Women's Open champion was solid throughout making short work of the par fives on the scenic French layout.
"I normally make my birdies everywhere apart from the par fives, so today was like a whole different day for me.
"I kind of hit it close to the pins on the par fives, which gave me more opportunities. No 3 was a really good shot as well there, but I didn't hit - well, I hit one really good hybrid the par-3 14th.
"Other than that, I hit a lot of hybrids and six irons and stuff today. Especially with some of the slopes on the fairways, it's quite hard to hit it at the pin and go for it every single time.''
Ko began on the 10th tee and mixed three birdies and a bogey heading out to make the turn in two-under par 33. She continued her momentum on the back nine and got to four-under par through 15 holes but she bogeyed the par four ninth to slip back to tied sixth.
The Kiwi, who defended her title at the Canadian Open to become the youngest and the second youngest winner on the LPGA Tour, is playing her seventh major championship.
She will continue her quest to become the youngest major champion overnight in the second of three rounds, after the tournament was shortened to 54 holes with Thursday's play was washed out.
American Morgan Pressel, who won the 2007 Kraft Nabisco Championship at the age of 18 years, 10 months and nine days, is the youngest women's major champion.