The first hole-in-one of her career lifted world number one Lydia Ko into medal contention as Charley Hull's hopes of securing a golden double for Great Britain faded in Rio.
Ko defied strong winds at Reserva de Marapendi to surge through the field with a flawless 65, which included an ace from 140 yards on the eighth in a front nine of 29.
The 19-year-old New Zealander could only manage nine straight pars on the back nine as the wind strengthened but that was enough to end the day in joint second place, two shots behind seven-time major winner Inbee Park.
South Korea's Park, who was under pressure to give up her place in the Games as she battles a thumb injury, carded six birdies and five bogeys to finish 11 under par, with Ko and American Gerina Piller - who has never won a professional tournament - on nine under.
Playing alongside Park in the final group, Hull carded an erratic 74 featuring four birdies and seven bogeys to finish five under par, four shots out of the medal positions. Team-mate Catriona Matthew fared even worse with a 77 to lie one over.
Ko's hole-in-one was not the first of the day, China's Xiyu Lin having aced the same hole earlier, but it was surprisingly the first of the two-time major winner's career.
"I could not believe it when the ball disappeared," she said. "I almost did not know how to react. I would have loved to have done a dance or jump up and down but I did not want to look stupid if it was just close to the hole.
"It was the perfect shot at the perfect time and to do that at the Olympics, it does not get any better than that. It puts the cherry on top.
"I had a (score of) six under in my head and to have done that is really cool. I did not expect the wind to be this strong so it feels more like 10 under. I said we'd been pretty lucky with the wind the past two days - I opened my mouth too early.
"It was blowing really strong out there but I've put myself in a good position going into tomorrow and still have a chance to be standing on that podium.
"We all know that there is a lot on the line at the end [on Saturday] but I think I've just got to take it on as just another day out there and just focus on the shot I have in front of me and have a lot of fun. To be in this position in my first-ever Olympics, I think it's cool enough being there."
Hull insisted her chances of emulating Justin Rose in winning gold had not disappeared, adding: "You don't really look at the gold being gone because it's not. It's only six ahead.
"You don't really think about playing for third because if you do and come up short, what's the point? You search for first place and if you come up short you've still won a medal.
"I'm tied for fifth and that's nothing going into the final round. Anything can happen."