Pak struggled to a 71 in Wednesday's opening round, a sluggish performance she blamed on getting little rest after winning the LPGA Championship on Sunday. But the native of South Korea bounced back to improve to 9-under 135.
Icher, in just her second year on tour, shot a 66 Thursday and owned the lead at 134.
Pak moved back in contention with the help of an eagle on the par 5 ninth hole, chipping in from 25 feet.
"I had better sleep, better rest," said Pak, who had eight birdies and an eagle in Thursday's round. "The first couple of days I struggled. I couldn't sleep and I was waking up in the middle of the night. But I had pretty good sleep last night and I feel great."
The 24-year-old Pak last week became the youngest player to win four majors. She also claimed the U.S. Women's Open in 1998 and last year's Women's British Open.
Icher played a bogey-free round to move into the lead, sinking a 40-foot putt for birdie on No. 11. But she does not feel any added pressure of trying to win the title on home soil.
"I do not have the fate of the world on me," said Icher, who credits a new driver for her improved play. "It's my second year on the tour, and I'm learning."
First-round leader Mi Hyun Kim of Korea shot a 70 Thursday and was alone in fifth place at 136.
Sorenstam shot a bogey-free round Thursday, including birdies on Nos. 15 and 18.
"I'm very pleased. I had a lot of chances, but the greens are still something I'm not used to," Sorenstam said. "It's been tough to make the transition from last week."