The 72-hole tournament is scheduled to end Saturday.
Haru Nomura made seven birdies for a 66 and finished two rounds at 6-under 136.
Michelle Wie was in a large group at 4 under that included Suzann Pettersen, U.S. Women's Open champion In Gee Chun, Lexi Thompson and Juli Inkster, who birdied her opening hole of the second round before the storms arrived.
Ko started quickly by opening with two birdies. But she birdied only one of the par 5s, though she never missed a fairway.
"I felt more comfortable today. I was pretty pleased to see all my balls finish in the fairway," Ko said.
Ko played with Morgan Pressel, who was 3 under, and Paula Creamer, who was even. They could tell storm clouds approaching and Creamer jokingly said that play would be stopped right before they finished. All they could do was laugh when they reached the ninth green to mark their golf balls.
"At least we don't have to go back to the range," Pressel said.
Ko is coming off a five-victory season that included her first major (Evian Championship in France) and LPGA player of the year.
She conceded to having some rust during her start on Wednesday, though she was dialed in for much of her second round until the rain arrived.
"Last year was an awesome year, so much better than I ever expected," Ko said.
"Things I only dreamt of happened. But you have to forget about it. It's a whole new year. Obviously, you come into the year with a bit of confidence, but you never know what's going to happen."
Pettersen started with a 73. After making the turn, she birdied five of the next six holes - the exception was the par-5 fifth hole - to get within range. Pettersen played with Stacy Lewis (1 over) and Alison Lee, with whom Pettersen was involved in a bitter rules controversy at the Solheim Cup last year over a putt that Pettersen said was never conceded. Pettersen said there was no drama with the group and the media "want to make it another story."
"Alison and I are pretty good friends," she said. "I'm very happy with the way I'm playing."
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