With Annika Sorenstam and Karrie Webb skipping the tour stop in preparation for next weeks made-for-television match with Tiger Woods and David Duval, Pak entered the event as the clear-cut favorite.
Pak, third on the money list, has already won three times this year. She birdied three of her first four holes at Squaw Creek Country Club before fighting an erratic driver for the remainder of the round.
McGuire is a New Zealander seeking her first LPGA victory. She birdied five of her first eight holes before playing the last 10 in even par.
Will had four birdies and an eagle on Friday. The highlight of her round came when she holed a 5-iron from 147 yards on the par-4 10th.
Donna Andrews, Becky Iverson and Laurel Kean are one shot off the lead at 4-under.
Jane Geddes is in a group at 3-under. She finished her round by acing the par-3 9th, her final hole of the day.
Defending champion Dorothy Delasin also opened in 3-under-par 69.
Full-field scores from the Giant Eagle LPGA Classic