"A little bit of everything wasn't really working well out there today, but it was tough conditions," the 25-year-old, who has never won the tournament, was quoted as saying by the BBC.
"I think I grinded (it out) really well out there."
Former U.S. Open champion Choi, hit six birdies on the way to matching her first-round 67 on the Old Course with her only bogey coming at the seventh.
She is one shot clear of Japan's Miki Saiki (66) and a further stroke ahead of overnight leader Morgan Pressel of the U.S. who is third on eight under after a 70.
Park has won the first three majors of 2013 - the Kraft Nabisco title, LPGA Championship and U.S. Open.
Officially, victory this weekend would not constitute a grand slam, since the Evian Masters in September has this year been granted the status of the fifth major.
But securing a fourth successive major in a calendar year would be heralded as an unprecedented achievement.
Park has already matched the feat of Babe Zaharias in 1950 of winning the first three majors of the year.
(Writing by Alison Wildey in London; Editing by Ken Ferris)
