"There has been a few things happened to me this year - back injuries, broken toe," said the South African, who played the Paris Olympics with a piece of one shoe cut away because of her toe injury.
But Buhai, whose two LPGA victories include a major title at the 2022 Women's British Open, said her form had been solid, and after two weeks off she was ready to attack the Arnold Palmer-designed TPC River's Bend course, which is hosting the tournament for the first time.
"I think I was smart with when I could attack," said Buhai, who had four birdies on the front nine and four on the back.
With her two-putt birdie at 18 she had a one-stroke lead over Thailand's Jeeno Thitikul and China's Liu Yan.
Eight players shared fourth on 67, a group that included world number one Nelly Korda and New Zealand's Lydia Ko, the Paris Olympics champion.
"There were a few pins out there where you had to still try to be aggressive, and the greens got a little firm and ran through, but I then made some good up and downs to keep me in it," Buhai said.
"I hit it great, putted well - that tends to add up to what it did."
Liu, who has missed the cut in her last seven starts and is searching for a first top-10 of the year, started on the 10th and had two eagles - at the 18th and at the eighth - in her 66.
"Today my driver was very good," said Liu, who played her last two holes in three-under.
