World No.4 Minjee Lee birdied the penultimate hole to match the four-under-par 68s of Grace Kim and Steph Kyriacou as Tiburon Golf Club came under attack.
Only 17 players in the elite 60-strong field for the $US11 million ($A17.1 million) Tour Championship failed to break par during the opening round in Naples.
With seven birdies and an eagle at the par-5 17th hole, Lee took full advantage of the benign conditions on Thursday (Friday AEST).
"It's an honour playing my first Tour Championship, and I don't think I was necessarily in a position to be nervous about anything," the leader said.
"I thought to myself, 'Whatever the score is or how I play, let's just kind of prepare for next year's season'.
"That kind of alleviated the stress in today's round to be able to get a better score."
The 26-year-old gave her chasers hope with a bogey at the last as American Allisen Corpuz emerged as Lee's closest pursuer, two shots back after a first-round 66.
Defending champion and world No.1 Jeeno Thitikul is lurking ominously at five under, alongside Japan's Nasa Hataoka and Korean pair Sei Young Kim and Jin Hee Im.
The season-long points leader and player-of-the-year favourite, Thitikul racked up five 5s on her colourful scorecard.
But after entering the first round having totalled a neat 50 under at the course over the past three years, the Thai superstar looms once again as the player to beat despite carrying a right wrist injury into her title defence.
"I know we are playing for $4 million cheques but it's just like almost a bonus for me this year to be able to play here," Thitikul said.
"I have had a really good run for the year, so nothing to think about that much."
A month after teaming to win the prestigious International Crown event in Seoul, Aussies Lee, Kim and Kyriacou all sit in a nine-way tie for seventh, four strokes off the lead.
Already the season's money leader, Lee could more than double her 2025 on-course earnings if she collects the record $US4 million ($A6.21 million) winner's cheque on Sunday.
The triple major winner made an inauspicious start, bogeying the second hole, before picking up five birdies in an otherwise mistake-free round.
"Yeah, pretty solid day today," Lee said.
"I was pretty steady all day, so nothing too crazy.
"But hopefully I can take the momentum from today into the next few days."
With some fine scrambling, Kim and Kyriacou also both had five birdies and just one bogey in their rounds.
New Zealand's world No.4 Lydia Ko opened with a three-under 69, two better than American world No.2 Nelly Korda, who remains winless in 2025 after saluting seven times last year.
Hannah Green, the fourth member of Australia's triumphant International Crown team, is propping up the field following a horror four-over 76, including a double-bogey six at the last.
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