A two-putt birdie from 30-feet on the long 2nd provided the catalyst for a telling early surge and Madsen made further gains at the 3rd, 5th, 6th, 7th and 9th.
Another birdie on the 12th took her to seven-under but she gave that shot back on the next hole with her only bogey of a terrific round.
“I like playing into the wind and the front nine really suited me,” added Madsen, who harnessed the conditions to fine effect with a display of poise and purpose.
“My putting was very good today, especially on that front nine and I was able to take advantage of the chances I had.”
Koo, the US Curtis Cup player, pieced together a tidy, bogey-free 68 as the 19-year-old revelled in her first experience of Scottish links golf.
“I’d been working on hitting stingers but there was one shot that was super skinny,” she said with a wry smile. “I was like, ‘oh, that is so bad’ but it ended up rolling along the ground to 10-feet and I holed the putt.”
Koo’s Curtis Cup team-mate, Anna Davis, birdied three holes in a row from the 10th in a 69 while Scotland’s Hannah Darling, a beaten semi-finalist in both 2021 and 2022, also opened with a three-under round to tuck herself in among the qualifying frontrunners.
“I’m just looking to keep the foot down,” said the former Girls’ Amateur Champion who is the top ranked GB&I player in the Nairn showpiece.
“I think if you get too cautious and try to protect your position, then that’s when it can get away from you. So, I’ll just go out there, hopefully post a similar round and get through to the match play.”