Trevin Parks was the best scorer (27 points) when the green-clad team also took home the second meeting at home, 87-76.
Södertälje turned an eight-point deficit in the third period and then never let go of the game.
The home team led the most with 15 points, 84-69, in the fourth period.
Södertälje won the basic series before Norrköping and Borås and is the favorite in the meeting with Jämtland which is decided in the best of seven matches.
The next match is played in Östersund on Tuesday.