Defending champion Hyo Joo Kim had a 61, making it the first time since the 2003 Kellogg-Keebler Classic that two players are double digits under par in the opening round.
"I don't think I've ever actually started a round with four birdies, so it was nice to take advantage of the good start and continue that on my back nine," Ko said.
"As every golfer, when things go well, you also think about the things that could go terribly wrong. I feel like I stayed patient and was rolling it really well."
A moderate start to her 13th season came to life on the Cattail course at Phoenix's Whirlwind Golf Club when the 28-year-old Ko got into a rhythm after a rare putter change and started piling up the birdies.
She said the idea of matching Annika Sorenstam's only sub-60 round in LPGA history, 25 years ago on a different course in Phoenix, entered her thinking when she birdied her 14th and 15th holes of the round after starting on No.10.
But she didn't hit a seven-foot birdie putt firmly enough on the par-5 seventh.
"That would have been nice to hole that one," Ko said.
"But who knows? Maybe if I holed that one I might not have holed the other two. You can't think about 'what if?' Birdied some other ones that I didn't expect, so kind of just evens out in that sense."
It was the ninth round of 60 or lower on the LPGA, the most recent by Lucy Li at Pinnacle Country Cub in Arkansas in 2024.
Kim also started on No.10 and shot 28 on the front nine, finishing birdie-eagle-birdie, including a hole-out from the fairway on the par-4 eighth hole.Â
World No.2 Nelly Korda, who opened the year with a 54-hole win in Florida and was runner-up last week in California, holed out from the 18th fairway for eagle in the middle of her round and shot 63, one of her career best.
The American is three shots behind in outright third.
Robyn Choi is the leading Australian, in a tie for 34th at four under, one stroke ahead of compatriot Karis Davidson and Thailand's world No.1 Jeeno Thitikul.
