Ireland's Leona Maguire has shot a dazzling nine-under 63 to catch Lydia Ko in the LPGA Tour Championship and set up a duel for the richest prize in women's golf.
At stake on Sunday at Tiburon Golf Club in Florida is $US2 million ($A3m) to the winner.
Maguire, one of 11 first-time winners on the LPGA Tour this year, is too far back in the points race to win LPGA player of the year, though $US2 million in a single day would make up for that. That would be nearly twice what Maguire has made in 23 previous events.
Ko had a five-shot lead to start the third round - seven shots ahead of Maguire - and made a mixture of birdies and bogeys that stalled her round. She fell one shot behind Maguire until chipping to tap-in range on the par-5 17th. She finished with a 70.
They were at 15-under 201 in what shaped up as a two-player race for the $US2 million.
No one else was closer than five shots to them. Former US Women's Open champion Jeongeun Lee6 had a 68 and Gemma Dryburgh of Scotland had a 69. They were at 206.
The forecast for Sunday was for rain and 32 kph gusts.
"Nothing I'm not used to from Ireland," Maguire said when she finished her round. "Looking forward to one last walk of the season tomorrow."
Brooke Henderson kept her slim hopes alive for player of the year. The Canadian, who had to withdraw last week with a back injury, had two eagles in her round of 65 and was at nine-under 207.
Henderson will have to win the CME Group Tour Championship and have Ko finish third or worse to win player of the year.
Ko came into the week with a one-point advantage over Australia's Minjee Lee for player of the year.
Lee, who won the US Women's Open and leads the LPGA money list, has fallen nine shots behind at six under after a 71 and is all but certain to be eliminated.
Ko is a lock to win the Vare Trophy for having the lowest scoring average.