The South Korean shot a six-under 66 at the Guadalajara Country Club to bring her to a 54-hole total of 15-under 201, just ahead of American Cristie Kerr, who posted 67 for 203.
Six birdies – three on the front nine and three on the back – put the 24-year-old Park out in front on the eve of Sunday’s final round.
And having won July’s Evian Masters in France and picked up the LPGA Malaysia last month she will be looking for more solid form on Sunday in a bid for a third title and to stay in the battle with Stacy Lewis in the Player of the Year points race.
Second-ranked Lewis, who grabbed her fourth 2012 title last week in Japan and is attempting to become the first US golfer to win the award since Beth Daniel in 1994, rounded up the day on a 71.
Park currently lags 58 points behind Lewis, meaning she will need to win this event along with the season finale next week to have a chance of passing her.
But she did not dwell on the pressure and instead expressed delight with her recent performances.
“That would be two big achievements this season,” said Park, who leads LPGA Tour money list. “Earlier this season, everything didn’t really work well, especially my putting.
“I was just trying to find the right stroke and right putter. I kept trying different putters and kept trying different strokes and just finally found my stroke. I felt really comfortable after that.
“I can’t expect Stacy to play bad two events in a row. Even if I don’t win player of the year, I’ve had a really great season and I’m really satisfied with myself,” she added.
South Korea's So Yeon Ryu ended the day in third spot after she carded a 67 for 204 while American Angela Stanford, who held a one-shot lead over Park going into the third round, finished with a 72 for 205.
Mexican star and tournament host Ochoa, who retired in 2010, shot three-under for 70 and is sharing 19th place while top-ranked Yani Tseng was one-over after a 76.
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- Stacy Lewis