With about half the field of 69 (who will all play the Old Course twice on Sunday) still to finish their first round, Karin led by two strokes from Ireland international Martina Gillen (Beaverstown), a golf scholarship student at Kent State University in America.
Martina highlighed her card with an eagle 3 at the 401yd, par-5 sixth and birdied the 10th, 12th, 14th and 16th in halves of 36 and 34. She had one bogey, at the ninth.
Miss Gillen comes straight from the American college circuit as do University of California team-mates Vikki Laing from Musselburgh and Ria Quiazon who were lying fourth and joint fifth respectively before the end of the day's play at St Andrews.
Vikki had a two-under-par 73, to be one while Ria, despite her lack of experience over links courses, was able to match the par of 75.
"We didn't fly in until last night," said Vikki "but we're not feeling too tired at the moment. Maybe it will catch up on us Sunday."
Four of Miss Laing's five birdies came at the par-5 holes and she would probably have birdied the par-5 18th, which is "only" 386yd but for driving into a fairway bunker and having to come out sideways.
Vikki birdied the sixth, 10th, 11th, 12th and 16th in halves of 38 (one over par) and 35 (three under). Her only bogeys came at the second and ninth.
Miss Quiazon will also be playing in the British women's open amateur championship at Lindrick, near Worksop, Notts during her golfing holiday in the United Kingdom.
Rachel Lomas, hot on the trail of a place in the England team for the home internationals at Cruden Bay in September, was joint runner-up behind Nathalie David, first French winner of the Helen Holm Scottish women's open amateur stroke-play at Troon at the end of April.
She did not make a splash in the recent English close championship but she was back in form here with a five-birdie, two-bogey round of 72 which held the clubhouse lead for an hour or two.
Miss Lomas birdied the first, sixth, eighth, 11th and 18th in halves of 36 (one under) and 36 (two under). Her bogeys came at the second and ninth.