The South Korea-born teenager, now competing under the New Zealand flag, had her worst round of the week, a level-par 72 to remain on eight under, but it was enough to edge her ahead of four players who stand a shot back.
Frustration for Ko came at the 18th hole where she recorded a bogey five to give up the chance of a two-shot cushion.
"I don't think I was more nervous than other days," she said. "I tried to have more fun, but my score wasn't as good. I mean, 72 is better than 73 or any other score, so I'm pretty happy.
"It's good to stay at the top of the leaderboard, but my first goal was to make the cut and hopefully top 15 or something. To be up there is just an honour, especially playing against the world's best."
She has a high pedigree cast of pursuers, with world number two Stacy Lewis and the South Korean trio of Jiyai Shin, Chella Choi and Inbee Park all on seven under. Choi had shared the lead at the start of the day but carded a one-over 73.
Lewis cracked 70 for the first time this week as a 66 lifted her into contention, and Ko accepts she is likely to feel a little pressure as she goes after what would be a historic title.
"A little bit, because I know they're all great players. Stacy Lewis is second in the world just trailing behind Yani (Tseng). I played with Stacy and I know she's a great player," Ko said.
"It's quite nerve-racking, but I'm really here for experience and fun, and I'm having fun at the moment. If I don't win tomorrow [Monday], I don't think I'll be disappointed. If I do win, I'll take it like an honour.
"It's kind of a thrill to see lots of people and especially them trailing behind.
"Fifteen-year-olds don't lead at an LPGA event all the time. I'm very surprised. But I've been playing really good golf and I've been really confident with my game.
"I'm just going to try my best. I've got to play my own game. I can't concentrate on what the other players are doing. If they shoot 66 and I shoot 68 and I lose, I can't control what they do. So I'm just going to play my game and have one shot at a time."
The youngest winner in the history of the LPGA is Lexi Thompson who was 16 when she captured the Navistar Classic last September.
Ko will be 15 years, four months and three days old tomorrow [Monday].
Americans Sydnee Michaels and Moira Dunn share sixth place on five under, with Scotland's Catriona Matthew the leading British player on three under after a fine five-under round of 67, quite comfortably her best of the week.
Tseng had led after the first round but continued the struggle that began with Friday's 75 as she went round in 74 to slide back to one under.