Buhai, a 30-year-old South African who has never won on the LPGA Tour, birdied four of the final eight holes to post 12-under 132.
“I’m trying not to keep thinking it’s a major. It’s just another tournament,” said Buhai, whose best previous British Open finish was a tie for 30th in 2017. “I just keep trying to do what I’ve done the last few weeks. I’ve kept the mistakes off the card the last two days.”
Alone in second at 9 under was 20-year-old Hinako Shibuno, a rookie on the Japan LPGA Tour who is making her LPGA Tour and major championship debut.
“I just wanted to make the cut. That’s all,” Shibuno said.
Brooke Henderson of Smiths Falls, Ont., is tied for 18th at 4 under after shooting a 1-under 71 on Friday. Hamilton’s Alena Sharp missed the cut at 4 over.
Shibuno, who shot 66 on Thursday, had a 69 on Friday and wowed spectators at Woburn Golf Club with her fearless play. She led for much of the afternoon before Buhai overtook her. Shibuno has two victories in Japan this year and is ranked 46th in the world.
American Lizette Salas was third at 8 under. She birdied the first four holes en route to a bogey-free 67.
“Awesome day,” Salas said.
Bronte Law, the top-ranked English player at No. 19, also shot 67 and was four shots back alongside Celine Boutier, second-ranked Sung Hyun Park, Caroline Masson and local favourite Charley Hull, who is playing on her home course. Boutier had the day’s lowest round at 66.
Danielle Kang had a disappointing 72 and was six shots back.
“I don’t know, nothing really felt like yesterday,” Kang said. “I guess today was one of those off-days and yesterday was the day that everything comes together. But I know what I’m capable of. Even if I’m playing bad, I know I can give myself opportunities.”
Defending champion Georgia Hall was also 6 under after a 69, along with Ariya Jutanugarn (70), Carlota Ciganda (69) and top-ranked Jin Young Ko, who was frustrated after a 70. Ko is seeking her third major title of the year after winning last week’s Evian Championship in France.