A teenage prodigy who has triumphed twice on the LPGA Tour, Pressel also carded five birdies and two bogeys to set the pace in the 54-hole event.
Swede Anna Nordqvist birdied three of her last seven holes for 67 to end the day level with American Ryann O'Toole, Canada's Alena Sharp and Taiwan's Min Lee.
Canadian teenager Brooke Henderson was among a group of nine who opened with 68s while defending champion Stacey Lewis, the American world number three, settled for a 72 after mixing an eagle with one birdie and four bogeys.
Pressel, who at 18 became the youngest winner of a women's major at the 2007 Kraft Nabisco Championship, felt her round owed a great deal to damage control.
"I really didn't put myself in a whole lot of trouble," the Florida resident told reporters. "I wouldn't say that I certainly had the best game out there. I felt like I made a few mistakes.
"But then I came down birdieing eight and eagling the last hole, that definitely helps."
Pressel, who has recorded two top-10s on the LPGA Tour this season, said her revamped game was still a work in progress.
"It regressed a little bit the last month or so, so I've kind of tried to go a little bit back to square one and really slow things down again, keep working on getting it where I'd like it to be," she explained.
"It's a hard thing to work on it a lot on the range and then take it to the golf course, so I've had to be patient with myself in that respect. But I feel better about my game."
South Korean world number two Park In-bee, the highest-ranked player in the field, returned a 71 while Michelle Wie, back in action after withdrawing from the Kingsmill Championship two weeks ago because of a hip injury, carded a 74.
(Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Larry Fine)
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