Park, the South Korean star who swept the first three majors of the season and has six LPGA Tour victories this year, had seven birdies and two bogeys at Royal Mayfair to match Kerr at 8-under 132.
Kerr, coming off the United States' loss to Europe in the Solheim Cup, shot her second straight 66. She won the Kingsmill Championship in May for her 16th career LPGA Tour title.
"It's a course where you have to really shape the ball off the tee and be creative," said Kerr, the 2006 winner at London Hunt in Ontario. "I played very well the last couple of days. It's unfortunate I bogeyed the last hole today, but that's what happens when you hit an errant shot and have to chip out to a difficult pin."
On the par-4 18th, Kerr drove into the left rough and was startled on her approach when a door on a portable restroom slammed shut.
"I mean, (the noise) really scared me," she said. "I could have hooked (the ball) in the water."
Park also bogeyed the final hole. She's returning to play after a break in South Korea following a failed bid in the Women's British Open to become the first professional to win four straight majors in one season.
"I worked on a couple of things after that because I wasn't hitting it my best at the British Open. I'm obviously hitting it a lot better this week and putting a lot better," she said. "On the greens, I was really comfortable because they're so pure wherever you hit it, it's just going to go right there."
Park holed out from 20 yards from a bunker on the par-4 first hole for a birdie.
"I always love to start with a birdie," Park said. "It's like a good luck charm for me."
She finished second last year at Vancouver Golf Club, three strokes behind teen star Lydia Ko.
Park and Kerr will play together for the third straight day Saturday.
"It's always fun to play with Inbee," Kerr said. "She always plays well, and I enjoy playing with her."
"Good putting made the difference today," Icher said. "The course is quite hard, (with) tricky fairways and big rough, so I'm looking to get some good drives and putts on the weekend."
Ko was 6 under after a 69. Last year in British Columbia, the New Zealand amateur star became the youngest winner in LPGA Tour history at 15 years, 4 months.
"I've just got to play my own game and hopefully get some more putts rolling in," Ko said.
Paula Creamer and 49-year-old Laura Davies also were 6 under. Davies had a 66, and Creamer shot 68.
"I'm holing putts," Davies said. "That's the difference. ... You know, you can't compete if you can't hole putts."
Charley Hull, the 17-year-old English player coming off a strong performance in the Solheim Cup, was 5 under after a 66. She played alongside Ko in the first two rounds.
"I played pretty solid," Hull said. "I wasn't hitting that good today on the front nine with my tee shots because I felt a little tired out there. The back nine I got something going, holed a few putts and I'm pretty pleased with that."
Michelle Wie missed the cut with rounds of 73 and 76. She won the 2010 tournament at St. Charles in Winnipeg, Manitoba.
"I'm completely under the weather," said Wie, also a member of the losing U.S. Solheim Cup team. "I was struggling today, just could not get out of bed. I just felt really sick today, though that is no excuse.
"I'm going to take advantage of the two days I'm not playing, and just not leave my bed and just kind of sleep for 40 hours straight."
U.S. teammate Stacy Lewis withdrew Thursday after an opening 74, citing illness.
Sweden's Caroline Hedwall, a record 5-0 in the Solheim Cup, was 4 under after her second straight 68.