Annika Sorenstam, who fired a 59 during the second round of this event in 2001, was two shots off the lead at 5-under-par 67 along with Grace Park, Kelli Kuehne, Dorothy Delasin and Patricia Meunier-Lebouc.
Pak played the back nine first and jumped out of the gate with a stretch of birdies. She drained a 30-footer at the 11th and knocked a pitching wedge to three feet at the 12th. At the par-5 13th, Pak two-putted for birdie to make it three in a row.
She faltered with a bogey at the par-4 16th but surged into the lead thanks to a brilliant second nine.
Pak birdied the first on the Championship Course at Moon Valley Country Club and hit her third shot from a bunker to five feet at the par-5 fourth for the first of three consecutive birdies.
The Korean then hit a 9-iron to 18 feet at the ninth to cap off a magnificent round.
'I knew my swing and everything was more consistent,' said Pak. 'Even last week I didn't have a good tournament, but I felt like the swing was good. I was really comfortable today. It was a lot of fun.'
Webb got things started with a 10-foot birdie at the second and added back-to- back birdies starting at the fourth. At the ninth, Webb landed a 6-iron within 25 feet of the hole and ran home the putt to make the turn at 4-under.
'I got off to a good start today,' said Webb. 'So, it was a lot easier just to trust it and not to worry about things too much. I think my swing is in a lot better shape than it was this time last year.'
The Australian birdied the 10th and knocked a wedge inches from the cup at the par-4 14th. She tapped in for birdie and moved to 7-under with a birdie at the 16th. At the closing hole, Webb failed to save par from eight feet to fall back to minus-6.
'I think my whole game was pretty solid,' said Webb. 'All in all today I'm very happy, obviously a little disappointed to bogey the last, but probably the two best swings of the day, a driver and a 9-iron in there, so I can't complain about that.'
Sorenstam, who is gearing up for her appearance at the Colonial on the PGA Tour, had six birdies and one bogey to finish within striking distance.
Angela Stanford, Giulia Sergas, Joanne Mills, Helen Alfredsson, Hee-Won Han, Kris Tschetter, Akiko Fukushima, Sherri Turner and Soo-Yun Kang share eighth place at 4-under-par 68.
Rachel Teske, who defeated Sorenstam in a playoff to capture this event last year, finished in a group at 3-under-par 69.