Here's where things stand through 54 holes in Rio:
Leaderboard: Park (-11), Ko (-9), Piller (-9), Shanshan Feng (-8), Charley Hull (-5), Amy Yang (-5), In Gee Chun (-5)
What it means: Park is 18 holes from winning the first gold medal awarded for women's golf since 1900. Up one to start the day, she stretched her lead to three with a trio early birdies but fell all the way back into a tie with Piller at 10 under when she played a nine-hole stretch, Nos. 6-14, 3 over. But a Piller bogey at 18 and a birdie-birdie-bogey finish for Park have the latter up two heading into Sunday. With her future uncertain and her retirement looming, perhaps sooner than later, this would be a fitting addition to the closing chapter of Park's career after she struggled through injury to qualify for the LPGA Hall of Fame earlier this year. The seven-time major champion and world No. 1 has already won four of the LPGA's five current major titles, and gold would put her in rarefied air. Perhaps the biggest challenge on Saturday will come from the apparent heir to her throne, Ko, the 20-year-old current No. 1 who's already claimed two majors and LPGA's Race to the CME Globe. Park and Ko will be joined in the final group by Piller, last year's Solheim Cup hero who posted 3-under 68 and remains in search of her first individual win.
Round of the day: After a sluggish first two days, Ko raced up the leaderboard with a bogey-free 6-under 66. She turned in 6-under 29 thanks to four birdies and the first hole-in-one of her life at the par-3 eighth. After the red-hot start, Ko battled stiff winds on the back nine to close with nine straight pars and position herself two shots off the lead.
Best of the rest: Australia's Su Oh made seven birdies, six on the front nine, for a 5-under 66. She sits tied for 10th at 4 under for the week, five shots back in the race for bronze.
Biggest disappointment: Charley Hull (74), Brooke Henderson (75), and Stacy Lewis (76) all started the day within two shots of the lead and all failed to break par. They now sit 5 under, 4 under and 4 under, respectively, through 54 holes. Separately, Denmark's Nicole Broch Larsen went from 7 under through 36 holes to 2 over through 54 after a round of 10-over 81 complete with eight bogeys and a quad. Finally, first-round leader Ariya Jutanugarn withdrew after 13 holes, citing a knee injury. The reigning British Women's Open Champion carded three bogeys, two doubles and three triples and was 12 over for the day when she called it quits.
Shot of the day: At 20 years old, Ko finally made the first ace of her life when she holed out from 140 yards at the eighth.
Not to be forgotten, China's Xiyu Lin made the first hole-in-one in women's Olympic golf history, also at the eighth hole and shortly before Ko.
That makes three aces at the eighth in these Olympics: Ko, Lin and, for the men, South Africa's Jaco Van Zyl.
Quote of the day: "I haven't been very lucky with hole-in-ones. ... I didn't really expect it to go in, but when it finally dropped, I thought was going to cry almost, you know cry from happiness." - Ko