Se Ri Pak was three shots off the lead at 7-under-par 137 alongside Akiko Fukushima.
'I'm very happy with my round today,' said Sorenstam. 'I'm also so happy because I feel better than I did on the previous couple of days. Yesterday, I felt like my game was good, but it wasn't sharp enough because I was so tired from being sick and not having a lot of sleep. Today, coming out here again made a big difference.'
Sorenstam, who is trying to become the first player since Kathy Whitworth in 1968 to win 10 times in a single season on the LPGA Tour, was four strokes off the pace to start the round at Seta Golf Course and began to make her move with a birdie at the par-5 first.
The defending champion ran into trouble with a bogey at the par-3 fifth but recovered with a birdie at the very next hole. At the par-3 ninth, Sorenstam ran home an 18-foot putt for birdie to make the turn at minus-5.
Sorenstam made it two in a row with a 20-foot birdie at the 10th and added a birdie at the par-4 13th to move to 7-under.
At the par-5 17th, Sorenstam reached the green in two and drained a 15-foot putt for an eagle. The Swede capped off her round with a birdie at the last to secure the outright lead.
'I know what it will mean tomorrow,' said Sorenstam. 'It's here in my head. I'll just have to be patient tomorrow and take it one shot at a time and not get too far ahead of myself. I've learned that lesson the hard way. But I'll be excited to go out there tomorrow.'
Pak, who captured the Nine Bridges Classic two weeks ago in her native Korea, had seven birdies, two bogeys and a double-bogey for a round of 69. Pak is seeking her sixth victory of 2002.
First-round leader Mi Hyun Kim struggled with four bogeys and three birdies to finish four shots off the pace at 6-under-par 138. Kim was joined by Rosie Jones, Jeong Jang, Yuri Fudoh and Kasumi Fujii in a tie for sixth.
Catriona Matthew, Janice Moodie, Sophie Gustafson, Jill McGill, Vicki Goetze-Ackerman, Midori Yoneyama, Mayumi Hirase, Woo-Soon Ko and Chieko Amanuma share 11th place at 5-under-par 139.
This is the last event in which players can earn money towards qualifying for the ADT Championship. The top-30 players on the money list after this tournament earn a spot in the season-ending event.
Full-field scores from Mizuno Classic