ST ANDREWS, August 2, 2013 (AFP) - Playing in the afternoon, when 20mph winds whipped across the Old Course, the slightly-built 25-year-old failed to be knocked off her stride, packing half a dozen birdies into the five under par round.
On 10 under par, she led by one from Japan's Miki Saiki, who had composed her best of day 66 in the early morning when the weather was much calmer if, at times, a little wet.
American Morgan Pressel, the joint first round leader, slipped into third place on eight under after a 70, while Norway's Suzann Pettersen was the top European on seven under following a 67.
Park Inbee's bid to make history by winning four majors in one calendar year is still alive, but will need to claw back eight shots on Choi over the final 36 holes.
Not quite at her robotic best, the 25-year-old Park had a 73 -- a birdie at the last making dinner taste a little sweeter.
"But whether I win or not, I've already had great moments here," she said. "I feel if I can handle this kind of pressure then I'll never be afraid of anything again."
Choi won the US Open last year, but admits she has struggled to kick-on from a first major victory.
"But my coach is here from Korea this week," said the player who has six top tens but no wins this season.
"I haven't seen him since February and the first question he asked is 'where has all your confidence gone?'"
The coach is Robin Symes -- an Irishman who runs a golf academy in Seoul -- and Choi first met him seven years ago when she was still in high school.
"I've got an Irish friend of Robin's (David Jones) as my caddie for a one-off this week and my manager, trainer and mental coach are also here so it's a real team effort," she continued.
Two massive birdie putts -- from 45 feet at the fifth and 35 feet at the 15th - stood out in her round.
"That's been the big difference," she claimed. "I have holed a lot of good putts over the last two days."
Saiki claimed her 66 -- she twice holed out for eagle twos -- was the best round of her career.
"Because it is a major and we are here at the Old Course and surrounded by so much history then it must rate as my best ever round," she said.
Saiki's highlights were easy to pick out. At the 371-yard fourth, she holed her eight iron second shot from 127 yards and at the 353-yard seventh, she repeated the trick with a wedge from 108 yards.
The cut fell on one over par and among those to miss out were the first round joint leader Camilla Lennarth (82 for four over), three-time winner Karrie Webb (76 for six over) and Tani Tseng (74 for two over), who won the title back-to-back in 2010 and 2011.
Leading second round scores of the women's British Open at St Andrews on Friday (par 72):
134 - Choi Na Yeon 67 67
135 - Miki Saiki (JPN) 69 66
136 - Morgan Pressel (USA) 66 70
137 - Suzann Pettersen (NOR) 70 67, Lee Jee Young (KOR) 70 67, Nicole Castrale (USA) 70 67
138 - Mikaela Parmlid (SWE) 69 69
139 - Ryu So Yeon (KOR) 69 70, Mamiko Higa (JPN) 70 69, Angela Stanford (USA) 69 70, Stacy Lewis (USA) 67 72, Park Hee Young (KOR) 70 69
140 - Ryann O'Toole (USA) 67 73, Lizette Salas (USA) 68 72, Shin Jenny (KOR) 69 71, Lee Meena (KOR) 71 69, Dori Carter (USA) 68 72, Paula Creamer (USA) 68 72, Lin Xiyu (CHN) 72 68
141 - Lee-Anne Pace (RSA) 70 71, Pernilla Lindberg (SWE) 68 73
142 - Catriona Matthew (GBR) 68 74, Mariajo Uribe (COL) 69 73, Candie Kung (TPE) 72 70, Yoo Sun Young (KOR) 71 71, Sydnee Michaels (USA) 67 75, Park Inbee (KOR) 69 73, Ji Eun Hee (KOR) 67 75, Katherine Hull-Kirk (AUS) 69 73