Thailand's Chanettee added seven birdies in her bogey-free effort at TPC River's Bend, where American Gigi Stoll shook off a late bogey with a birdie at the last to post a seven-under 65.
Chanettee's round was highlighted by her eagle at the par-four 10th, where she hit a "perfect shot" with a nine iron from the fairway that found the bottom of the cup.
"After I hit, I just felt like, 'Oh, this one is going to be close to the pin,'" she said.
"I didn't expect it's going to roll to the pin!"
The 21-year-old won in Portland in 2023 and captured the Dana Open last year, but she's been frustrated during a 2025 campaign that included a tie for 43rd at the Evian Championship followed by a missed cut at the Women's British Open.
After feeling in recent weeks "like I cannot hit the ball", she checked in with her coach last week and on Thursday had a confidence-boosting performance to show for it.
"Actually, today I played really good," she said, adding that she was solid with every club in her bag.
"I just want to continue to hit like this until the end of the year."
Stoll, chasing her first LPGA title, eagled the par-five 11th and was seven-under before a bogey at 17.
She rebounded with a closing birdie for a 65 that put her one stroke clear of a group of six players sharing third on 66.
That included 12-time LPGA winner Kim Sei-young of South Korea, who teed off on 10 and had three birdies in her first nine holes before she racked up five birdies and two bogeys coming in.
"I played quite solid, especially back nine," said Kim, who was joined on six-under by compatriots Kim A-lim - who eagled the 18th - and Park Kum-kang, along with Thailand's Patty Tavatanakit, Taiwan's Chien Peiyun and American Jenny Bae.
World number two Nelly Korda carded a five-under par 67 to headline a group of four sharing ninth that also included Australian Gabriela Ruffels, Bianca Pagdanganan of the Philippines and Spain's Julia Lopez Ramirez.
Thailand's world number one Jeeno Thitikul opened with a three-under 69, and defending champion Lydia Ko of New Zealand carded a 70.