Ko sits at eight-under and leads countrywomen In Gee Chun and Jeongeun Lee6 by one stroke at the Old American Golf Club in Dallas.

American Emma Talley and South Korea's Min-G Kim shot 65s and head into Friday two shots back, with six players tied for sixth at three shots back, including Australia's Sarah Jane Smith.

Defending champion Angela Stanford is tied for 33rd after an opening 69.

Ko shot par on each of her first seven holes until an eagle on her next, the par-5 17th.

She then birdied six of the next eight holes en route to a bogey-free round.

"It's been a while, bogey-free round," said Ko, who fell to No.2 in the world after Nelly Korda won the Women's PGA Championship to move into top spot.

"I had great some shot and great putts."