Duncan tapped in a four-foot par putt on the final hole in Belleair, Florida, on Sunday (Monday AEDT), and there was a long line of players waiting to douse water on someone who has endeared herself over the years for remarkable perseverance amid a battle of self-doubts.

The 35-year-old is the oldest first-time winner on the LPGA Tour since Leta Lindley was 36 when she won the Corning Classic in 2008.

It was the perfect time for Duncan to turn in a great week. The American was No.74 in the Race to CME Globe, and victory moved her to No.34 and among the top 60 who qualify for the season-ending Tour Championship next week in Naples, Florida, where the winner gets $US2 million ($A2.95 million).

Three Australians playing in Belleair - Grace Kim, Cassie Porter and Karis Davidson - also confirmed their places at the end-of-season event.

With compatriot Hannah Green (11th) having already qualified, Porter clinched 52nd place, Kim 53rd and Davidson 55th to secure their spots in the 60-player field.

Kim was the leading Aussie at the Belleair tournament, finishing tied for 26th at three under after rounds of 66, 67, 74 and 70.

Davidson (68, 71, 69, 71) tied for 35th place at one under, with Porter (68, 67, 73, 76) finishing 58th at four over.

Duncan closed at 16-under 264, two shots ahead of South Korea's Choi (68) who had to settle for her third runner-up finish in her past three tournaments. Choi has gone 127 starts on the LPGA without winning.

"I have so many people to thank that believed in me when I wasn't sure I wanted to keep playing or if I was good enough to keep playing," Duncan said. 

"To be here, I'm completely shocked."

Duncan began the week with a birdie-birdie-eagle start, making a hole-in-one on the third hole at Pelican Golf Club. Far more important was her finish on a gorgeous day in the Tampa Bay area.

She fell two shots behind when Choi birdied the 11th and Duncan's shot into the par-3 12th rolled back off the severe slope and into the water. But she hit wedge to four feet from the drop zone to salvage a bogey.

Choi had a three-putt bogey on the 13th, and two holes later the hard-luck South Korean went long of the par-3 15th, chipped from beyond the green and had to scramble for bogey to fall into a tie.

Duncan took over from there with two shots she won't soon forget.

Her approach to the 16th was well right of the pin and went up the back slope, narrowly missing a sprinkler head and rolling back down a slope to six feet for birdie and a one-shot lead. 

She drove into a bunker on the 17th and faced a shot to a pin just beyond a deep bunker. It came out beautifully to 20 feet and Duncan holed that birdie and took a two-shot lead, which proved the final margin.

Haeran Ryu (66) and Yuri Yoshida (68) tied for third. 

American Nelly Korda closed with a 67 and finished seventh, six shots behind. Korda needs one more win to qualify for the LPGA Hall of Fame, her next chance coming in the season finale in Naples.

- With AAP