Norrköping started briskly and had a 33-28 lead after the first period.
But the final place would not be secured so easily.
Nässjö came back strong in the second, and at half time it was only one point that separated the teams after Nässjö's David Höök was 55-54 with a three-pointer with five seconds left.
The Smålanders would pull away from something in a tight and low-scoring third period that the home team won 15-10, and the lead was then six points big.
The ending would be dramatic.
In just over two minutes, Norrköping managed to score eight straight points, and was then only one point away from equalizing with six minutes left.
And three minutes later, at the position 78-78, Phillip Carr stepped forward and put an important third for Norrköping who would not let go of the lead again.
Norrköping won 89-86, and is ready for the final after 4-2 in matches.
Final opponents will be either Jämtland or Södertälje who will meet in a seventh decisive semifinal tomorrow.