That left the diminutive 23-year-old Ernst a stroke in front of South Korea's Lee Mi-hyang, with twice major winner Cristie Kerr lurking among a group of six who opened with 68s.
Level with LPGA veteran Kerr were her fellow Americans Brittany Lincicome, Jennifer Song, Gerina Piller and Jaye Marie Green, and South Korean Kim Sei-young.
Lurking three strokes back was world number one Ko, after the New Zealander birdied three of her last seven holes to move into contention for her sixth LPGA victory of the season.
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Ernst, who won her first LPGA title in a playoff for last year's Portland Classic, finished in style with a birdies at the par-five 17th and par-four last, where she nearly holed out with a five-wood approach from 192 yards.
"It almost went in," a smiling Ernst told reporters after hitting 17 of 18 greens in regulation. "The crowd got pretty wild like it rolled right over the edge. "So that was nice ... to have a little tap-in birdie."
Ko, back in action this week after taking a fortnight's break, is in pole position to clinch the LPGA Player of the Year award, the money list title and retain her number one ranking, and was in good position for all three after opening with a 69.
However, South Korea's Park In-bee, a winner of two major championships this season, is in close pursuit and she launched her bid for further LPGA honours by carding 71.
Park stands second in both the money list and the Player of the Year points standings, and leads the way in the hunt for the coveted Vare Trophy with a scoring average this season of 69.433, just ahead of Ko (69.449).
Either Ko or Park would clinch The Race To The CME Globe, which carries a $1 million bonus, with victory this week where the winner's cheque is for $500,000.
"Even if it doesn't exactly go the way I would love it to by the end of Sunday, I think I've just got to be pleased and proud with the way this year has gone," said Ko, 18, who won her first major crown at the Evian Championship in September.
(Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Larry Fine)
