The second-ranked South Korean star hit a 7-iron to 3 feet to set up a birdie on the par-3 16th, and made a 6-foot birdie putt on the par-5 17th.
"I was able to make a lot of putts and I was able to control the shots a bit better today," Park said. "It's a definitely a plus, because the last couple of days I've been making a lot of mistakes."
She had a 10-under 206 total at tree-lined Club de Golf Mexico, where she struggled with distance control in the high altitude the first two days.
"Today was a bit cooler, but you just get used to it," Park said. "It's not so much about the weather as it is about the altitude. It's about the distance control."
Park is making her first start since withdrawing in China two weeks ago because of a cyst on her left middle finger. She has four victories this season, winning two of the five majors.
Sei Young Kim, also from South Korea, was second after a 70. She won the event two weeks ago in China for her third victory of the season and has wrapped up the tour's rookie-of-the-year award.
"On this course, tee shots are very important, so I'll try to focus on them," Kim said. "If I keep it in the fairway, I should play well."
Kim beat Park on the first hole of a playoff in April in Hawaii, holing out from 154 yards for eagle. Kim forced the playoff with an 18-foot chip-in for par.
"She's a tough player and she really gets other players going," Park said about Kim. "She's very competitive and passionate and it's always good to play with a player like that in the final round. ... She's a fighter and she makes those shots and putts where she needs them."
Australia's Minjee Lee, the second-round leader, had a 75 to drop into a tie for ninth at 3 under.
"It was just crazy. We just couldn't believe it," Ciganda said. "It was too much traffic and we didn't know what was going on."