Birdies at the third and fourth holes were followed by bogeys at the seventh and 13th before the 20-year-old launched her attack. She nailed a long, curling birdie putt on the 14th hole before showing outstanding distance and then touch with a stunning chip-in for eagle at the 15th, joining Nordqvist, Altomare and Jutanugarn at the top of the leaderboard.
"It was probably the shot of the year for me, the chip-in on 15," Ko said. "It was really cool to play here, and I feel like the support was immense. I played my heart out and I tried my best and that's all I can do."
Ko provided further evidence she is rediscovering her best form after a miserable year by her lofty standards.
Last week she finished runner-up at an LPGA Tour event in Indianapolis and said the feeling of being in contention had given her confidence this week. She had missed the cut at two of her three previous tournaments and hadn't finished better than 20th since mid-June.
Fresh off her eighth top-three finish at a major, Ko will be the headline act at the next Tour event, the New Zealand Women's Open teeing off on September 28 at Windross Farm, near Ardmore.
- AAP