Mikaela Parmlid, Soo-Yun Kang, Lorie Kane, Shi Hyun Ahn, Janice Moodie, Lorena Ochoa, Angela Stanford, Hee-Won Han and Marcy Hart are tied for fourth at minus-5 at Berkleigh Country Club.
Lunke, who sank a 20-footer on the 18th hole of a playoff against Angela Stanford and Kelly Robbins to win her U.S. Women's Open, started on the back nine Thursday. She drained an 18-foot birdie putt at the par-5 13th then kept up her strong play on the par-5s.
At 16, Lunke chipped to 20 feet and made the putt. She missed the green with her second shot at 18, but chipped her third to 2 feet and converted the birdie try to make the turn at minus-3.
Lunke continued her fine form on her second nine. She rolled in an 18-foot birdie putt at the second, then holed a pair of 12-foot birdie putts at seven and nine to join the leaders.
'I didn't expect it,' said Lunke. 'I haven't been hitting the ball that great. I was really glad that last one went in.'
Lunke has struggled badly since her major breakthrough. She has not come close to the winner's circle and has barely been able to make cuts. But the 25-year-old thinks improvement is right around the corner.
'I know my game is better than it was last year,' said Lunke. 'I've gotten a little bit longer and more solid. This is a rebuilding year for myself to try and regain composure for everything that went on last year.'
Diaz vaulted to the top of the leaderboard with an amazing run on her first nine, the back side at Berkleigh Country Club. She birdied five in a row from the 13th, including four from inside 15 feet.
She parred her next eight holes before a 3-foot birdie putt at eight. Now Diaz, who had surgery on her ankle in December, is in the hunt for her first victory since the 2002 LPGA Corning Classic.
'It feels pretty good, but it's not 100 percent,' said Diaz. 'Mentally, as well as physically, I wasn't ready when I came back, but I think the only way to prepare is to keep playing and dig through it.'
McGill was not flawless in her round the way her fellow co-leaders were. Like Lunke and Diaz, McGill opened play on the back nine and birdied all of the even-numbered holes except the 10th.
She started her second nine with a bogey at the first when her drive went left and her approach went long. McGill rebounded with a chip-in birdie from 40 feet at the second.
McGill laid up short of the green with her second at the fifth, then knocked a wedge to 10 feet. She made that birdie putt, then rolled in a 14-footer for birdie at the eighth to post the first 66.
'Today was enjoyable,' said McGill. 'It was very mellow for me and I needed to do what I had to. I have played well here before, so it's nice to come back here.'
Tournament host Betsy King and Nabisco Championship winner Grace Park are part of a group tied for 13th at minus-4.
Defending champion Candie Kung, Se Ri Pak, Karrie Webb, Beth Daniel and Dottie Pepper, who is retiring at the end of the season, are part of a logjam tied for 29th place at 2-under-par 70.