And another eagle-birdie finish - for the second straight day - made it happen for the 21-year-old from Thailand.
Thitikul won the record-setting $US4 million ($A6.12 million) first-place cheque by capturing the CME Group Tour Championship in Florida on Sunday.
It's the biggest money prize in women's golf history, bigger than even the winner's shares in three of the four men's major championships this year.
Thitikul shot a seven-under 65 on Sunday and finished the week at 22 under, one shot ahead of American Angel Yin (66). Yin had a two-shot lead walking to the 17th tee, only to wind up settling for the $US1 million runner-up check.
The win and the massive first prize came down to the 18th hole, Thitikul and Yin tied at 21 under after a back-and-forth day atop the leaderboard - both knowing a mistake would likely come with a $US3 million cost.
Thitkul had a two-shot lead after three holes; Yin had a two-shot lead with two holes left. Neither was safe; Yin birdied and Thitikul bogeyed the par-4 fourth for a tie at 16 under, and Thitikul eagled the par-5 17th to pull into a tie with Yin at 21 under.
They both hit the fairway on 18. Thitikul's approach was nearly perfect, stopping about 5 feet from the cup. Yin's response stopped maybe 15 feet away, giving Thitkul the edge as they walked up the fairway.
Yin's birdie putt just missed. Thitikul's was dead centre. And history was hers.
She already had clinched a $US1 million bonus this week through the Aon Risk-Reward Challenge, a competition based on how players score on a designated hole each week.
In the end, it wound up as a whopping $5 million ($A7.65 million) week for the youngster - and going eight under over the four days on the Nos. 17 and 18 at Tiburon Golf Club made it happen.
New Zealand's Lydia Ko was third. five shots behind Thitikul, after a closing 63.
Hannah Green led the Australian contingent in the elite 60-player field, finishing tied 18th at 10 under after a 69, with Grace Kim (68) tied 25th at eight under, Minjee Lee (68) tied 30th at seven under and Gabriela Ruffels (68) tied 35th at six under.
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