The midfielder broke the deadlock in the 67th minute at the Stade Chedly-Zouiten in Friday's encounter as the hosts crashed out of the competition.
Realistically, Al Ahli Benghazi needed maximum points against the Tunisians and from their final pool match at ES Tunis.
Defeat means Jozef Vukusic's men are four points adrift of ES Setif with just one match remaining.
However, three points for Sfaxien means they can now look forward to the semi-finals as Es Tunis - who have matches to play - are unable to catch their tally of 10 points.
Neither team was able to find the breakthrough in a tense first half.
But Sassi made the difference shortly after the hour mark, bagging the only goal of the game, much to the delight of head coach Philippe Troussier and his side.