Annika Sorenstam remained one stroke behind Estill as she also carded a 70 to get to minus-14. Vicki Goetze-Ackerman fired a 5-under 67 to climb to 11-under-par 205 and LPGA Tour rookie Shi Hyun Ahn is one stroke behind her at minus-10 after a 6-under 66.
Estill opened with a two-putt birdie at the par-5 second to move to 14 under. She then birdied the par-5 fifth for the third straight round.
The 1991 PING-Cellular One LPGA Golf Championship winner picked up her third birdie at the eighth after knocking her approach to 4 feet.
Things quickly took a turn for the worse. Estill knocked her second shot over the green at the ninth and had to take an unplayable lie penalty. She pitched on the green and two-putted for a triple-bogey.
Estill came right back to birdie the 10th from 15 feet. She faltered to a three-putt bogey at the 13th at Corning Country Club to return to minus-13.
Estill, whose last top-10 finish came in 2001, dropped in a 12-foot birdie putt at the 15th and closed with a 4-foot birdie at the last that gave her the first 54-hole lead of her career.
'It's exciting to be here to be honest with you,' Estill said. 'I have kind of enjoyed the whole experience today, a roller coaster a little bit, but to be honest, it was great to be here and I had a great time.'
Estill, who owns one win, has 26 top-10 finishes. That pales in comparison to Sorenstam, who owns 50 career LPGA wins and is a Hall-of-Famer.
'Annika's a great player and she's not going to ease up on you,' said Estill. 'Today was a gift. I don't think I have seen her hit it like that for a while, so I have to be prepared tomorrow to go out there and hit again.'
Sorenstam had an up-and-down round. She rolled in a 7-foot eagle putt at the second to tie Estill in the lead at 14 under. However, she faltered to three bogeys over a four-hole stretch from the sixth.
Around the turn, the top player in the women's game returned to her solid form. Sorenstam drained a 28-foot birdie putt at No. 10 and came back to birdie the 12th to get to 13 under.
Sorenstam chipped in for birdie at the 13th, then sank a 5-foot birdie putt at the 16th to get to minus-15. She missed an 8-foot par putt at 16 to fall one behind Estill.
'If you look on the positive side, I am in contention for tomorrow,' said Sorenstam. 'I have a chance to win another tournament, so overall, this is going to make me feel happy, but I am not too proud of that round. I have a chance to win tomorrow, that's all that matters.'
Natalie Gulbis and Denise Killeen posted matching rounds of 2-under 70 on Saturday. They share fifth place at 9-under-par 207. Carin Koch, Kelli Kuehne, Nadina Taylor and Laura Diaz are one stroke further back at minus-8.
Juli Inkster, the defending champion, struggled to a 1-over 73. She shares 29th place at 3-under-par 213.