Park has led since the second round and made five birdies and one bogey to stay on course for victory.
The 28-year-old Park reminded everyone this week why she deserved to be the LPGA'S youngest hall of fame qualifier with an exceptional Olympic performance after bouncing back from injuries.
China's Shanshan Feng was five strokes behind Park, trying to protect her silver medal position.
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Several golfers who are stroke or two behind Feng who could be in medal contention include world number one Lydia Ko of New Zealand, Japan's Harukyo Nomura, Britain's Charley Hull, and Americans Gerina Piller and Stacy Lewis.
The Olympic format cannot end in a tie and calls for a playoff in the event of players finishing level in any of the medal positions.
Russia's Maria Belikoeva, boosted by a hole in one on the fourth, fired a 62 on a Saturday, an Olympic record.
With grey clouds looming, the field of 59 players did a double tee-off Saturday on the first and 10th holes to avoid bad weather.
Thailand's Ariya Jutanugarn, who was clubhouse leader after the first round, withdrew on Friday with a knee injury.
(Editing by Ed Osmond)
