Late surge lifts Thailand's Atthaya Thitikul to LPGA Queen City lead
Thailand's Jeeno, who announced last month she preferred to be known by her longtime nickname, had seven birdies in her second straight six-under par 66 at TPC River's Bend, which is hosting the event near Cincinnati for the first time.
New Zealand's Ko, who followed up Olympic gold in Paris with a victory in the Women's British Open last month, had an eagle and four birdies in her six-under round.
Ko had set the early target, but Jeeno squeezed past with a birdie at her final hole, the ninth - one of the toughest on the course.
"You have water on the front, and then if you go long it's going to be in the rough and a downhill lie," Jeeno noted.
"But just commit to the shot," she added. "I had pitching wedge, so it's kind of easier than the practice round when I had, like seven- or eight-iron."
The Thai made a belated start to the season after battling a thumb injury but teamed with Yin Ruoning to win a third LPGA title in June in the Dow Championship pairs event.
She started the day one stroke off South African Ashleigh Buhai's lead and picked up four birdies in her first nine. After a bogey at the 10th she added birdies at the par-five sixth and eighth before her final flourish.
Ko had more ground to make up, but produced her second straight bogey-free round to shoot up the leaderboard.
The highlight was an eagle at the eighth, where Ko watched playing partners Charley Hull and Rose Zhang hit it close.
"I had missed the green," Ko said. "So I was like, man, I want to at least make up and down for a birdie. I hit it exactly the way that I was envisioning. As soon as it came off the club face and landed Charley said 'good shot' and, like, five seconds later it went in the hole."
Ko, who said this week that her big wins last month were "kind of like being punched in the face twice in a very good way," said taking advantage of the course's reachable par-fives was key.
"I didn't really make that many mistakes. Obviously that's crucial around a golf course like this when people are shooting five-, six-under throughout the field."
South Korean Ryu Hae-ran was in third place on 10-under 134 after a seven-under 65 that included an eagle from off the green at 11 and six birdies -- five of them in a burst from the fifth through the ninth holes.
"I like today," Ryu said.
Mexico's Maria Fassi and Gaby Lopez were among a group of four players sharing fourth place on nine-under 135, along with American Yealimi Noh and China's Liu Yan.