The Swede, who is hosting the Ladies European Tour event for the last time, started on the 10th hole in rainy weather. She parred the first 10 holes, then reeled off four birdies. She also birdied the last two holes.
Johanna Head of England, who was 7-under until she bogeyed the 17th and 18th, shared second at 67 with Anna Rossi of Italy.
Jill McGill, the only American LPGA Tour player in the field, shot 69 and shared fourth place with four others.
Sorenstam, who will stop playing in December to start a family and focus on her business and golf interests, had never played a competitive round on the course. Its only an hours drive from her parents hometown north of Stockholm, where she grew up and honed her game.
A big gallery followed Sorenstams group despite the rainy day.
Impressive, I would not have watched golf in this kind of weather, she said. It was fun to see so many people out there today and that so many of them stayed on the course. After all, it took us five hours to complete the round.
The 37-year-old winner of 10 majors posted one of her best scores of the year despite the miserable weather.
Its my lowest round in a quite a while, she said. Ive had more bogeys than birdies in the last two and half months, so this was really nice. You have to be patient in this kind of weather. Its difficult to keep the grips dry. But the greens were superb and I putted extremely well.
Sorenstam missed the green on the last par-3 hole, but she hit the flag with her chip and the ball bounced in.
Sorenstam won the tournament six times from 1997-2006, when it was named the Compaq Open, HP Open and Scandinavian TPC Hosted by Annika.
Catriona Matthew of Scotland is not defending her title in Sweden.
Helen Alfredsson of Sweden, the LET money leader who finished runner-up in the U.S. Open and recently won the Evian Masters, also skipped the tournament. She will play an event in Sweden hosted by hockey star Peter Forsberg.
Sorenstam will make her final competitive appearance in Europe on Sept. 5-7 in Denmark before making her last start at the Dubai Ladies Masters on Dec. 11-14.