The home side scored early in the first half with a goal from Peter Ndlovu.
Sundowns kept in front for most of the game until the 82nd minute when the Tunisians equalized through a header from Chadi Hammami.
Former Orlando Pirates midfielder Blaise Lelo-Mbele scored late in the match with a low shot to put his side into the final.
When Sundowns played Sfaxien in Tunisia in August they lost 4-0.