At 7-under-par, Sorenstam is tied atop with Tina Fischer.
The two sit one stroke ahead of Leigh Ann Mills, Marianne Morris and Tracy Hanson, while six others, including Wendy Ward, are just two back following scores of 67.
For Sorenstam, the round comes as her first at the State Farm since 1994.
She didnt seem like such a stranger on Thursday, however.
I played really well, claimed Sorenstam following her round. Im in great shape. There are still three more days, but its nice when you can get off to a great start.
She most certainly did that, as the 30-year-old rattled in two birdies and an eagle over her first nine holes of the day (the back nine), and then proceeded to birdie four more in a row from Nos. 1 through 5.
A lone bogey at the par-3 7th set back her round just slightly, but still, she finds herself in a familiar position entering Day 2 ' the lead.
I know its a good field this week, and Ive got to keep it going, Sorenstam said. Ive got momentum, Ive got confidence.
Those words should come as warning for the rest of the field.
In 19 events this season on the LPGA Tour, Sorenstam has won six times, her most recent coming in her last start, the Canadian Women's Open two weeks ago.
In addition, she has finished runner-up on four other occasions and third once.
Sorenstams year comes as a stark opposite to Morris. In her last nine events, Morris has missed seven cuts.
Nevertheless, the 36-year-old sits right in the middle of the action heading into Fridays play.
Its always fun to shoot 66, she said. I havent done that in a while.
This weeks event is the last of the State Farm LPGA Series race.
If it werent enough, Sorenstam leads that one too. She is 100 points better than anyone with 430 points.
To the winner this week goes 300 points and to the champion of the Series goes $100,000.
Click Here for for Full-Field Scores from the State Farm Rail Classic.