RANCHO MIRAGE, April 5, 2014 (AFP) - Thompson's effort in perfect morning conditions on the Dinah Shore Tournament Course at Mission Hills gave her a seven-under par total of 137.
She was joined atop the leaderboard at the halfway stage of the first women's major of the year by Pak, who had two birdies in a steady two-under 70.
The two were one stroke in front of American Michelle Wie, who had two birdies and one bogey in her 71 for 138.
First-round leader Feng Shanshan of China was tied for the lead after a birdie at the par-four seventh, her 16th hole of the day.
But she closed with back-to-back bogeys for a one-over 73 that left her tied for fourth with Cristie Kerr on 139.
Feng, whose 2012 LPGA Championship victory made her the first Chinese golfer to claim a major title, finished with four bogeys and three birdies. Kerr carded a 70 to get to five-under.
Pak, winner of five major titles, is seeking her first Kraft Nabisco Championship win.
Thompson, 19, is seeking the first major victory of a career that saw her become what was then the youngest LPGA Tour winner in 2011 -- at the age of 16. She added two more titles to her resume since then.
After a disappointing day on the greens on Thursday, Thompson said she "figured out some things" about her putting and iron play.
She felt her opening five pars were a slow start, but Thompson, who teed off on the 10th, came alive after that. She picked up birdies at 15, 17 and 18, then added five more on her inward run.
"I was just trying to stay in the moment and focus on each shot, not really think about what I was shooting," Thompson said.
"I had the same confidence over every shot, just committing to my line and just being confident over every shot or every putt, and I just tried to do that the whole way throughout, even at the beginning of the round."
Pak was pleased with another efficient, if unspectacular round.
"It's the same as yesterday," she said. "Everything has just been really solid. I kept it in fairways and from the fairway on the greens.
"I had a lot of opportunities today, but putting was not as good as yesterday, but still, really just a really smooth, solid round today."
Leading second-round scores on Friday in the LPGA Kraft Nabisco Championship, first women's major of the year (USA unless noted, par-72, a-denotes amateur):
137 - Lexi Thompson 73-64, Pak Se-Ri (KOR) 67-70
138 - Michelle Wie 67-71
139 - Cristie Kerr 69-70, Feng Shanshan (CHN) 66-73
140 - Catriona Matthew (SCO) 72-68, Anna Nordqvist (SWE) 71-69, Morgan Pressel 70-70
141 - Mo Martin 73-68, Amy Yang (KOR) 68-73
142 - Gerina Piller 77-65, Charley Hull (ENG) 73-69, Carlota Ciganda (ESP) 73-69, Azahara Munoz 72-70, Sandra Gal (GER) 72-70, Ryu So-Yeon (KOR) 70-72, Chella Choi (KOR) 70-72, Shin Ji-Yai (KOR) 69-73
143 - a-Minjee Lee (AUS) 75-68, Christina Kim 74-69, Angela Stanford 74-69, Stacy Lewis 73-70, Lydia Ko (NZL) 73-70, Karrie Webb (AUS) 73-70, Choi Na-Yeon (KOR) 72-71, Mirim Lee (KOR) 71-72, Jennifer Rosales (PHI) 69-74