Dordrecht struggled to find a way back into the game, but almost forced an equaliser early in the second half when a cross from Jordy van Deelen had Willem II keeper Kostas Lamprou scrambling across his box to collect.
The visitors threatened again on 56 minutes when Adnan Alisic's strike bobbled narrowly wide of Lamprou's goal.
However, Dordrecht fell further behind on the hour mark when Kurto failed to hold Samuel Armenteros's shot and Sahar was on hand to bundle home the rebound.
Anthony Biekman came off the bench to pull back a late goal for the away side, but it proved only a consolation as the Tilburg outfit held on for victory.
The result leaves Dordrecht eight points adrift at the foot of the table, while Willem are four points behind seventh-placed PEC Zwolle.