Thompson, who won her first major title at last year's Kraft Nabisco Championship, held the clubhouse lead for much of the day before being caught at the top by compatriots Lizette Salas (69) and LPGA rookie Alison Lee (66).
Taiwan's Hsu Wei-ling (69) was a further stroke back at eight under, level with Americans Jaye Marie Green (69), Gerina Piller (65) and Kris Tamulis (68), and South Korean Q Baek (68).
World number 13 Thompson, who has won four LPGA titles, was delighted to benefit from an early start after contending with firming greens on Thursday afternoon when she opened with a 69.
"I got a few more putts to roll in today," she told Golf Channel. "The greens were rolling really well out there early in the morning so it was nice.
"I played in the afternoon yesterday and it got pretty windy, especially on my second nine. The greens were just kind of baked out, they were bouncing out a lot. It was hard to get distance control down."
Thompson's only regret on Friday was not maintaining momentum after covering her front nine in a scorching six-under 30.
"I wanted to make more birdies on the back nine," she said. "I hit it in the rough on a few holes when I needed to hit into the fairway, but I'm very happy with my round."
South Korean world number one Park In-bee, who lost a playoff for last year's Meijer LPGA Classic to her compatriot Mirim Lee, improved on an opening 70 with a 68.
"I got off to a really good start the last couple of days but just couldn't do anything on the back nine," Park said after mixing four birdies with a bogey to finish five strokes off the pace.
"That's been the problem. I feel I'm rolling the ball better. I've just got to hit it a little bit closer and give myself a lot of opportunities over the weekend, then you just never know."
(Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Gene Cherry)
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