The final act was a 50-foot putt over a green that looked more like bunny slopes, three ridges to cover. The American nearly holed it, then rapped in a three-footer for par to polish off a clean card. Duncan's one-under round in Belleair, Florida, on Saturday (Sunday AEDT) took her to 13 under for the tournament.
Along with trying to win, Duncan still is not eligible for next week's season-ending Tour Championship, where the winner of the top 60 players who qualify gets $US2 million. A strong finish, even without winning, gets her there.
Three Australians - Grace Kim, Cassie Porter and Karis Davidson - all remain firmly in the mix to make the end-of-season event.
With compatriot Hannah Green already qualified, Porter is currently 47th, Kim 51st and Davidson 54th in the live rankings in the race to secure a spot in Naples, Florida.
Kim is the leading Aussie at the Belleair tournament, tied for 26th at three under after following rounds of 66 and 67 with a disappointing four-over 74 on Saturday.
Davidson (68, 71, 69) and Porter (68, 67, 73) are a shot further back in a tie for 30th, a distant 11 shots behind the leader.
Duncan admitted it was a challenge trying to handle the conditions.
"Today was a really tough day with the wind," she said. "The greens firm up, so I was really just trying to hit the ball in spots where you could at least hit the green or have a reasonable chip."
Hye-Jin Choi was tied for the lead until she missed the 18th green to the left and did well to get down in three for a bogey and a 70.
Duncan, who made her Solheim Cup debut at age 35 and is making her 222nd career start on the LPGA Tour, was at 13 under 197. Her lone birdie was a seven-iron to 18 feet on No.12.
Greens are the defence at the Pelican Golf Club, and some of the pins were in their toughest positions. Carlota Ciganda, who at one stage shared the lead with Choi and Duncan, three-putted five times, including the 18th.
American Nelly Korda also three-putted on the 18th for a bogey that gave her a 71 and left her six shots behind Duncan. Korda can qualify for the LPGA Hall of Fame with a victory.
Japan's Yuri Yoshida, who won her first LPGA title in the recent Asia swing, played alongside Korda and had a 67, leaving her two shots back.
Ina Yoon of South Korea (69) was three behind, and double major winner Haeran Ryo went from eight shots behind to four back with a Saturday-low 65.
- With AAP
