Angela Stanford and Gloria Park share third place at 4-under-par 67. Defending champion Meg Mallon followed at 3-under-par 68 along with Jeong Jang and Young Kim.
Birdie Kim began on the back nine at Highland Meadows Golf Club and struggled early with a bogey at the par-4 12th. She recovered with a birdie at the par-4 15th and tallied back-to-back birdies starting on the par-5 17th to reach minus-2 around the turn.
The 23-year-old added a birdie at the par-3 second and kept on rolling with a birdie at the par-3 sixth. Kim then birdied each of her last two holes for her share of first.
'I played with some really, really good players,' Kim said. 'I just tried to copy from them. Every hole, I hit the middle of the fairway and the green.'
Kim has adjusted well so far to her standing as a major champion, and the new status it has given her on the LPGA Tour.
'I feel free,' she said. 'I don't have to worry about going back to Q-school.'
Morgan also stumbled early with a birdie at the second, but she responded well with two straight birdies from the third. Morgan then birdied the sixth and added birdies at eight and nine to make the turn at 4 under.
The 30-year-old from Wales added a birdie at the par-4 12th after her second shot stopped 7 feet from the cup. She picked up a birdie at the par-5 18th to join Kim atop the leaderboard.
'I putted really well today,' said Morgan, whose best finish this year is a tie for seventh at the Wegmans Rochester LPGA. 'I hit a lot of good shots, but I really putted well.'
Stanford collected three birdies over her first nine holes. She cooled down on the back nine with a lone birdie at the par-5 17th to finish two shots off the pace.
'Overall I feel really good,' Stanford said. 'I only hit eight fairways, but every time I hit it in the rough I always had a chance to at least get it around the green.'
Park birdied the 11th, her second, but gave that shot back with a bogey at the 12th. She also bogeyed the 14th, but bounced back with two consecutive birdies from the 17th.
The 25-year-old bogeyed the first, but ran off four birdies over her last six holes.
HSBC Women's World Match Play Championship winner Marisa Baena posted a 2-under-par 69 to join Brittany Lang, Hee-Won Han, Nicole Jeray, Anna Acker-Macosko, Candy Hannemann, Johanna Head, Carri Wood, Leta Lindley, Sung Ah Yim, Shani Waugh, Amy Hung and Marilyn Lovander in a tie for eighth place.
Amateur Morgan Pressel was one shot further back in a group at 1-under-par 70.