Her 36-hole total of 130 smashed the tournament record by four shots. The old mark was originally set by Laura Davies in 1996 and later matched by Michele Redman in 2000, Moira Dunn in 2001 and Meg Mallon last year.
'I'm feeling good,' said McGill. 'I feel relaxed and calm. I'm still trying to have the same focus that I did yesterday, realizing that the end result is not what's important. It's the process.'
Suzann Pettersen posted a 9-under 63 and joined Soo-Yun Kang, who carded a 66, in second place. The duo is knotted at 11-under-par 133 at Berkleigh Country Club.
They will have to play strong golf on the weekend if they are to catch McGill.
McGill parred her first four holes, but broke into red figures at the par-5 fifth. She hit a 4-iron to the back of the green and two-putted.
Her play around the turn is what vaulted her to the top of the leaderboard and into the tournament's record book. At the eighth, McGill sank a 6-foot birdie putt, then collected back-to-back birdies with a 3-footer at nine. McGill made it three in a row at the 10th when her sand-wedge stopped 5 feet from the hole.
McGill was in trouble at the 11th but saved par. Immediately after that, she returned to her birdie ways. She rolled in a 20-footer at the 12th, and tallied a pair of 3-foot birdie putts at 13 and 14.
McGill parred three in a row from the 15th, but padded her record at the closing hole. She missed the green with her second shot at the par-5 hole, but chipped to 3 feet and kicked in the short birdie putt.
'When the green light is there, give it a go,' said McGill, referring to her strategy. 'If it doesn't work out, it doesn't work out. You can't judge what happens before it happens.'
This is all part of McGill's new outlook. She has not lived up to career expectations that started as an amateur. McGill won the 1993 U.S. Amateur Championship and the 1994 U.S. Public Links Championship. She finished as runner-up in the 1994 Amateur to Wendy Ward, but McGill has yet to find the winner's circle on the LPGA Tour.
'I'm not going to walk around 18 holes with a pit in my stomach,' said McGill, who received some advice from Nancy Lopez on the driving range Friday. 'I'm not going to wake up in the morning and almost dread what I am going to do. I am just going to go out and hit the first fairway tomorrow and the hit the first green.'
Pettersen had an impressive round of 10 birdies and one bogey, while Kang's card was a little more diversified. She recorded nine birdies, three bogeys and six pars.
Grace Park shot a 6-under 66, while Shi Hyun Ahn posted a 67 on Friday. The two are tied for fourth place at minus-10.
First-round co-leaders Hilary Lunke and Laura Diaz each carded rounds of 3-under 69. They are tied with Lorena Ochoa, who shot a 68 in the second round, for sixth place at 9-under-par 135.
Ward (66) and Mikaela Parmlid (69) are tied for ninth at minus-8.
Defending champion Candie Kung posted a 3-under 69 and is part of a group in 23rd at 5-under-par 139.
The 36-hole cut came at even-par 144 and among the notable players who failed to make the weekend were: two-time winner this year Cristie Kerr (145) and Women's British Open champion Karen Stupples (147).