Minjee Lee and Grace Kim both fired four-under-par 68s to trail An by four shots after the opening round as the world's premiers golfers hunt a record $US4 million ($A6.18 million) winner's pay day in Naples.
Lee and Kim are tied for 11th in the elite 60-player field, with Gabriela Ruffels and world No.5 Hannah Green a further shot back at three under and equal 17th.
Trying to end an uncharacteristically winless season in style, Lee went bogey-free around the majestic Tiburon Golf Club on Thursday.
But so too did the 28-year-old An, who accrued eight birdies to grab a one-shot lead over American Angel Yin.
"Today my putt felt really good," An said. "The speed was good and the shape was good. I just try to focus a little bit more."
An has never won on the LPGA Tour, but she finished 29th in the points race thanks to four top-10 finishes. Now she is in great position to land a massive victory.
Though a $4 million ($A6m) pay cheque is enticing, she said her attitude this week is that a tournament is a tournament.
"I think that mindset is good to focus during my play," she added.
Fellow Americans Marina Alex and 2023 Women's US Open champion Allisen Corpuz share third after starting with six-under 66s.
New Zealand's Olympic champion and former world No.1 Lydia Ko, who, like Green, has enjoyed a three-win 2025, is lurking ominously in a six-way tie for fifth at five under, three shots off the pace.
"The course isn't easy," Ko said. "I set a goal of shooting three under today, and somebody shot eight under. I was like, 'OK, maybe I need to make a few more birdies.'Â
"It's a course that can get away from you as much as you can shoot some low scores, so I'm just trying to stick to my game plan and go from there."
But world No.1 Nelly Korda has much ground to make up if she's to crown her seven-title year in typically glorious fashion after opening with an even-par 72 to be languishing in joint 44th spot.
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