Ko started the day four off the lead of China's Liu Yu, but a bogey-free round of seven-under par 65 at Wildfire Golf Club carried her to a total of 22-under par 266 and a one-shot lead over Liu, Spain's Carlotta Ciganda and American sisters Jessica and Nelly Korda.
Ko, who seized the tournament lead with birdies at 14, 15 and 16, was on the practice green keeping warmed up for a possible playoff with Liu, who arrived at the 72nd hole at 22-under but made her only bogey of the day to cap a two-under par 70 for a 21-under total of 267.
"I don't need to practice anymore!" a delighted Ko told well-wishers when the victory was secured, later adding that she had played the whole round just trying to stay in the moment.
"I didn't think about winning today," she said. "I had to really focus on my game. I just tried (to play) shot by shot and not thinking about the future, just, like, now. So, amazing day," she said of her first victory on US soil, after titles at the 2017 KEB Hana Bank Championship in South Korea and the 2018 Women's Australian Open — part of her 2018 Rookie of the Year campaign.
Liu was trying to become just the second Chinese player to win on the LPGA tour after Feng Shanshan, who has nine titles.
She started well enough with birdies at the second and fifth, but on a day when birdies abounded, she wouldn't find another until the par-five 15th.
That tied her with Ko's clubhouse lead, but she was unable to get past, or force a playoff, as her par attempt at 18 slid past the edge of the cup.
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