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The 12m putt down and across the slope with the perfect pace, hitting the flag and finding the bottom of the cup to be met by raucous applause from the biggest gallery on course.
The eagle catapulted the World No 1 amateur into the tournament lead and was the defining moment of today's second round at the New Zealand Women's Open in Christchurch.
"It was a turning point for me," said Ko, the only Kiwi to make the two-round cut. "I wanted a couple of birdies before that but they just didn't go in the hole. But then that 12m putt, it just drained in and it kind of made up for today."
From there Ko never looked back. The teenager will begin tomorrow's final round on six under and in a share of the lead with Korean Seon Woo Bae, who climbed 33 places with a remarkable eight under par 64 that broke the course record.
Italian Giulia Sergas (68) climbed to third and overnight leader Australian Nikki Campbell (71) dropped to fourth place.
Ko, who began today on the 10th tee in a share of fourth place, followed up her eagle with birdie on the 16th then added another on the second to get to six under for the tournament.
The only display of any weakness was when she flubbed a chip on the fifth, but she made a 15 foot save for par in nonchalant fashion.
"I was happy with that save. I got a really bad lie with the chip and I didn't know where that ball was going to go. I wasn't that happy about that hole even though I made the par."
The birdies continued on the following hole before she dropped her first shot of the day with a three-putt bogey on the seventh.
"[It] wasn't because I hit it badly - it was a short putt mistake so it is not something serious. I am hitting the ball well.
"I am pretty happy. I had a struggle the last three holes but I made par on the last two so that was OK. I don't think that there was something huge to complain about and I made an eagle so it was pretty good."
She parred her way in to head into tomorrow with a golden chance of becoming the first Kiwi to win the event. With that possibility in the forefront of her mind, Ko knows there will be nerves standing on the first tee in the final group.
"[The nerves] will be there tomorrow but I just have to control my nerves and that is what happens every time. I have been playing pretty good so hopefully I can play like I have the last two days. It was nice to see a lot of me out there today. I definitely felt their support."