American Stacy Lewis (70) was 10 under, one stroke ahead of Amy Yang (65), Jessica Korda (69) and Austin Ernst (70).
All of them are chasing Ko, who is chasing history as well as her sixth career LPGA victory.
"There are so many great players that are like one to four shots [back]," Ko said. "You just never know what's going to happen. I'm just going to concentrate on my game, stay really positive and if somebody else shoots a much bigger score than I do, I can't really do much about it. I'm just going to focus and hopefully I'll be able to shoot a good score."
By mid-round yesterday, much of the focus had turned to Ko. She three-putted the 10th and 11th, falling four strokes behind leader Jang. Ko slammed her putter into her bag and mumbled to herself, "OK, you've got to start working again". She outplayed everyone on the course from there, showing the poise of a tour veteran instead of the nerves of a teenager.
"She's acting more mature than 17 years old, like at least 25," said Choi, who played a practice round with Ko near Orlando last week. "She's so mature. Sometimes, when we're having dinner or lunch, she's 17 years old. She likes to eat some chocolate or ice cream, or that kind of stuff.
"But on the course or range, when she has a golf club, I think she totally changes."
It showed when she responded to those consecutive bogeys with a lengthy birdie run - five in a row.
Ko can finish second or third and still supplant Inbee Park, who is tied for 17th at 3-under, as the world's No1 player.
It would be the latest feat on Ko's growing resume, which includes winning the inaugural Race to the CME Globe, which included a US$1 million ($1.38 million) bonus last season, and becoming the youngest to win the tour's Rookie of the Year award.
Now, she has a chance to really make a mark.
"It's never over until you pull the glove out of your pocket and put it in your bag," Ko said.
