Park ran off six birdies in 10 holes to race to the top of the leaderboard, only to stumble coming back in for a 3-under 69 that left her two shots out of the lead.
"Felt like a roller coaster today," Park said.
Stacy Lewis, the former No. 1 who would love to be a spoiler at St. Andrews, hit her stride on the back nine with a 31 that gave her a 67 and a share of the lead midway through the opening round with Na Yeon Choi, Nicole Castrale and Mi-Jeong Jeon.
Paula Creamer played bogey-free for a 68.
"Once the round started, and especially playing so good the first few holes, that really gave me a lot of confidence," Park said. "I didn't feel much pressure during the round. I'm just glad that it is already started and I got the first round under my belt."
The conditions were perfect for scoring on the Old Course, with light rain falling from a lead gray sky and barely any wind. Some of the biggest names in women's golf, including Park, Lewis and Creamer, teed off in front of the Royal & Ancient clubhouse with no more than a few dozen fans in tow.
Park already has won three majors this year. No golfer, male or female, has ever won four in one year. The 25-year-old South Korean said she was more nervous than usual before getting to the first tee, perhaps because of all the chatter about a Grand Slam.
It sure didn't show.
Park opened with a wedge into about 7 feet for birdie, and then she really poured it on with an amazing display of her putting stroke. She rolled in a 30-foot birdie putt on No. 3 with perfect pace. She hit a hybrid the perfect distance on No. 4, giving her a flat line between ridges for an 18-foot birdie putt. She made a 35-foot birdie putt on No. 6, another birdie just inside 20 feet on No. 8 and a sixth birdie on No. 10 after a wedge that stopped 5 feet from the hole.
Just like that, there was a feeling of inevitability about this Women's British Open, much like it was for Tiger Woods when he won the British Open for the first time at St. Andrews by eight shots to complete the career Grand Slam in 2000.
Dressed in a black rain suit, her emotions never changed, even when she started to unravel.
It started with a tee shot into thick grass to the right of the 12th fairway and a shot he hacked out to the front of the green, below a 3-foot ridge some 75 feet away. Park left the putt some 15 feet short, and made that to save par.
But she couldn't save herself from another poor drive on the 13th, and her second shot came dangerously close to a large gorse bush. She chipped to 15 feet and made bogey, her first of the day. A delicate pitch-and-run helped her avoid another bogey on the 15th after a tee shot to the right.
Her biggest mistake came from the only bunker she found, just short of the 16th green. The ball was a few feet from the lip, and Park was tempted to take it up over the high face of the vetted wall. Instead, she turned sideways and blasted out to the middle of the double green, the ball about halfway between the holes cut for No. 2 and No. 16. From some 90 feet away, she didn't hit it nearly hard enough, and her par putt from 15 feet caught the lip.
She also three-putted the 17th from 40 feet when her first attempt came up 10 feet short. Considered one of the best putter on the LPGA Tour and looking for a short time as though she couldn't miss Park had consecutive three-putt bogeys and had lost four shots in a five-hole span.
"I thought that I fixed my problems coming into this week. I was hitting it so good on the practice round and I didn't really miss any balls," Park said. "I thought I was really prepared, but those couple of bad shots really shocked me. I couldn't really concentrate on the greens when I hit those shots. I've learned my lesson. Good thing I've got my time to fix that today and tomorrow."
She still was only two shots behind, opening with a round that could have been much better. Even so, Park had no complaints.
"A little disappointing, but I'm glad that I've done that in the first round instead of the final round," she said. "I'm looking to improve the next three days."
Only four months ago, Park and Lewis were battling for No. 1 in the world until the South Korean left everyone in her wake by adding to her major championship collection. The Kraft Nabisco Championship put her in position to take No. 1 away from Lewis, and the playoff win at the LPGA Championship and four-shot win at the U.S. Women's Open brought her to the brink of something grand.
Lewis, however, has some history of her own at St. Andrews. In her final event as an amateur, Lewis was part of the Curtis Cup team that beat Britain & Ireland at the home of golf. Lewis won all five of her matches. She turned pro and played in the final group of the U.S. Women's Open at Interlachen, which Park rallied to win for her first major.
Lewis and Karrie Webb were on the 11th tee when they saw Park make birdie to reach 5 under through eight holes.
"We both looked at each other and shook our heads," Lewis said. "We knew she was going to be there, but it's like she keeps doing it over and over and over again. The front nine is usually where you score. For me, it was the opposite. And I feel lucky to get away with one today."