Kiwi superstar Ko and Korda - who have both held the number one ranking - were at 11-under par after 72 holes.
The American started the day four ahead of Ko, led her by six shots at one stage, then fell apart, the nadir being a double bogey on a par three after a wretched tee shot.
Incredibly, Ko suddenly had a three-shot lead over Korda and Megan Khang as she prepared for the final hole, thanks to a slightly fortuitous hybrid on the 17th that appeared as a stroke of genius.
The ball ended up next to the hole giving Ko, playing in the group ahead of Korda and Khang, a simple birdie putt.
“To be honest, I thinned it a little bit and thought it was going in the water,” Ko said.
It could have been a shot that would help define her career given its Hall of Fame significance, but Korda had other ideas.
The tall American came up with a series of gems to finish with eagle and birdie, as she had done in the third round.
Ko saved the first playoff hole, after an awful approach shot, with her trademark magic chip.
But a poor putt on the next hole opened the door for Korda, who completed her ninth victory.
Ko, who has 20 victories, was typically philosophical, saying to dwell on poor crucial shots could be pointlessly “endless”.
Who knows what significance this day will hold down the line?
If Ko does nail the final point she needs to make the Hall of Fame, all will be forgotten.
If she doesn’t, this will turn into a day of frustrating memories, even if Ko appears to handle the ups and downs of professional golf with a wonderful perspective.
