Carin Koch posted a 4-under 68 to climb into third place at 8-under-par 136. Mhairi McKay, Sherri Steinhauer, Laura Diaz, Natalie Gulbis, Denise Killeen and overnight co-leader Liselotte Neumann are one stroke further back at minus-7.
Estill came out on fire. She dropped a pitching-wedge within 12 feet of the cup on No. 10, her first and drained that putt. She came right back with a 6-foot birdie putt at the next.
The 1991 PING-Cellular One LPGA Golf Championship winner made it three in a row when she drained an 18-foot birdie putt at the 12th. Estill stuck a 6-iron within 3 feet to set up her fourth straight birdie to open her round at the 13th.
Estill, whose last top-10 finish came in 2001, stumbled to a bogey at the 16th at Corning Country Club. She came right back with a 20-foot birdie putt at the next to get back to minus-9.
She sank her sixth birdie of her opening nine at the 18th, this one from 10 feet out. Estill missed the green right at the par-4 first and was unable to save par.
Estill erased that mistake as she drained a 30-foot eagle putt at the second to get to minus-11. She two-putted for birdie at the par-5 fifth. The 41-year-old sank a 15-foot birdie putt at the ninth, her last, to move ahead of Sorenstam.
'Well I hit it just as good as I did Thursday, and I got off to a really good start, but today I just made a lot of putts,' said Estill, whose best finish this year was a tie for 40th last week at the Sybase Classic. 'Today I made a couple of long putts. I made some good putts coming in on the backside. It was a lot of fun out there.'
Sorenstam, who shared the overnight lead with Neumann, also began her day on the back side. She pitched her second shot to the par-4 10th green to 9 feet and sank that birdie putt to get to 8 under.
The Swede got up-and-down for birdie from over the green at the par-5 12th. Sorenstam came back with an 8-foot birdie putt at the 15th to get to minus-10.
Sorenstam hit a poor shot off the first tee and that led to her first bogey of the day. She came back at the next with a two-putt birdie to erase that mistake. Sorenstam, who has won twice this season, knocked a 4-iron on the green at the par-5 fifth.
She calmly rolled in her eagle putt to jump to minus-12. She faltered to a three-putt bogey at the eighth, but Sorenstam stuck a 9-iron within 5 feet of the cup for birdie at the ninth.
'I'm quite pleased with my round,' said Sorenstam, who carded a 5-under 67 on Friday. 'Maybe not as solid as Thursday's, but I thought the wind was quite tricky at times as it was gusting in both directions, and I had a good feel for where the wind was coming from, but it was more of a crosswind on certain holes.'
Kelli Kuehne, Gloria Park, Nadina Taylor, Rachel Teske, Michelle Ellis and Vicki Goetze-Ackerman share 10th place at 6-under-par 138. Wendy Ward and Laura Davies lead a group of players one stroke further back at minus-5.
The cut line fell at 1-over-par 145. Seventy-nine players advanced to the weekend, however among those missing the cut was reigning U.S. Women's Open champion Hilary Lunke. She stumbled to a second-round 7-over 79 to fall to plus-2.