Ko currently sits in a tie for 15th place, four shots behind leaders Lexi Thompson and Jennifer Kupcho from the United States.
Kupcho, who has been a runner-up in each of the last three years on the LPGA Tour while still trying to capture her first title, ran off four birdies in a six-hole stretch around the turn for a 64 that set the target at 11-under 129.
"I don't know if I necessarily have it figured out," said Kupcho, who had 12 birdies over her last 28 holes. "I'm just hitting really good iron shots and getting myself in good position on the greens."
Thompson played in the afternoon and finished strong, her only blemish down the stretch a bogey on the par-4 eighth hole. She has not won since the ShopRite LPGA Classic more than two years ago and has slipped to No. 13 in the women's world ranking.
Nine players were separated by two shots at the halfway point at Pelican Golf Club.
Leona Maguire of Ireland cooled slightly from her opening 62 and had a 68 to join Wie-Ling Hsu (64). The group another shot back included Mi Yang Lee (62), Matilda Castren (64), world No. 1 Nelly Korda (66) and defending champion Sei Young Kim (67).
"I think it's going to be probably a shootout at the weekend," Maguire said. "If that's kind of the case, just trying to make as many birdies as you can and keep your foot down, see where it stands at the end of the weekend."
Jin Young Ko, who has won four of her last seven starts on the LPGA, had a 66 to get back into the picture. The South Korean start was five shots behind.
- with AP