Delacour teed off early and played much of her round in steady rain, but she fired nine birdies without a bogey at Columbia Edgewater Country Club to lead by one over American Gina Kim, a 23-year-old former amateur standout whose bogey-free 64 was her best ever LPGA round.

World number two Nelly Korda headed a trio on seven-under 65 that also included Sweden's Linn Grant and Germany's Olivia Cowan.

Delacour was untroubled by the morning's steady rain, opening with a birdie at the first hole then stringing together five straight birdies from the third through the seventh.

"When you're in that zone you don't realize your score and you're just not scared of anything," said Delacour, who added birdies at 14, 15 and 17. "You go to every pin and you know it's going to be fine."

Delacour missed just two greens and needed only 26 putts and said even her rare mistakes proved easy to rectify.

"I hit 16 greens, and when I was missing them I was pretty easy up and down," she said.

Kim, playing in Portland for the first time, said her opening birdie at the 10th hole was key to her entire day.

"I snap hooked my drive almost to the tent," she said. "But the fact that I stayed calm - I was like, you know what? We've still got a shot out of here.

"Took it out into the fairway. Had a seven-iron in. Had about 35 feet. Sunk it in, and I think it's really telling that golf is a funny game," Kim said. "It doesn't have to look perfect. As long as you are shooting the scores, that's all that matters."

She added birdies at 14, 15 and 18, then picked up four more in her last nine holes.

"I'm really excited," said Kim. "It's been tough this whole season, and I kept grinding through it."

Korda said the wet weather actually made it easier to attack the course.

"It boosts your confidence in a sense when you're hitting it good," she said. "Obviously, it's tough to play in the rain and there is a lot more going on, as in trying to stay dry and it's a little bit more hectic, but overall it didn't really dump on us."

Korda said regaining her confidence after a back injury sidelined her earlier this year has been a painstaking process.

"I played really well before it, and then when I came back it was very hard to get into the groove of things," Korda said. "I came back to pretty much four majors in a row, so definitely tough golf courses, firm golf courses where it's kind of hard to get your confidence back.

"But one step at a time I think. I'm pushing in the right direction."