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The Kiwi former world no 1 is tied for a two-shot lead with American Danielle Kang after shooting a seven-under par 64 at the Marathon Classic at Highlands Meadow Golf Club in Sylvania, Ohio.
Ko, who shot eight birdies and a bogey, says her approach play was what helped her rocket out to a strong start.
"I hit a few shots that was really tap-in range," Ko said after her round. "So when you have three or four of those opportunities it definitely puts a little stress off the putter which was kind of the bit I was struggling with last week."
The 23-year-old said she worked with new swing coach Sean Foley on being more aggressive, which seemed to pay off on the course.
"For me being aggressive is like, even if I am playing safe, I'm still being aggressive and committed over that shot," she said. "I think that's really important for me to be really committed and I'm hitting each shot with my 100% authority and I know I hit it better that way.
"It's good to be back in this competitive atmosphere to kind of put that into play to see what you need to get better at.
"The confidence and being aggressive is something that I'm continuing to work on and is something that really helps me a lot out there."
Ko also said playing in front of no crowds, in what was the second LPGA Tour event since the Tour was suspended due to Covid-19, took some getting used to.
"Obviously we love seeing the fans," she said. "We're gutted that they're not here this year but hopefully next year they'll be back. But right now safety is priority and staying healthy."
Meanwhile, fellow Kiwi Ryan Fox has shot a four-under par 67 to be four shots adrift of the lead at the European Tour's English Championship.
Danny Lee is also in action at the PGA Championship major in San Francisco.