Ji was also hot early, and a pair of birdies early on the back nine gave her a 66. They were at 13-under 200.
Brooke Henderson of Canada was poised to make it a three-way tie at the top until she bogeyed the par-3 closing hole for a 69, leaving her one shot behind. Nelly Korda had an eagle and four birdies for a 65 and was two shots off the lead.
Ko closed with seven straight pars.
"I felt like there were a few putts I thought I holed and it didn't go in," she said. "But I holed a really long one on 8 - that one, I probably didn't deserve - so everything balances out.
"It felt like I gave myself quite a few good looks and just wasn't able to hole in. But at the end of the day, all I can do is put a good roll on it and just give myself good opportunities out there."
Now she has an opportunity to start 2019 with a victory, something that took her until April to do last year as she began to emerge from a slump.
The LPGA Tour season opener is limited to winners each of the last two seasons for a US$1.2 million purse. It also includes a 49-player field of celebrities and athletes competing for a US$500,000 purse. Former Atlanta Braves pitcher John Smoltz had 39 points in the modified Stableford format, giving him a four-point lead over retired pitcher Mark Mulder. Mardy Fish from the world of tennis was five points behind.
Ko has been enjoying the interaction with athletes.
"You don't know what time it is out there. You're just having a really good time," she said. "I've never been caught up in what position I am and what I need to do. I probably need to do that a little bit more [in the final round] so I know what I need to do coming down the stretch, or the whole day, really."
