Bradley Grobler scored the only goal of the match just before the break. AmaZulu had Rhamson Zhuwawo red-carded in the first-half.
The home side had the most touches on the ball in the opening minutes with the midfield but it was the visitors who fired the first shot at goal in the sixth minute through Lebogang Manyama, but his shot was well-held by Philani Zikalala.
In the 33rd minute AmaZulu's Ayanda Dlamini went on a run on the left before cutting in to switch the ball to his right foot, but his curled attempt went wide of the far post.
Then six minutes later the hosts were reduced to ten-men when midfielder Rhamson Zhuwawo was shown a straight red card as he was adjudged to have kicked out at Grobler.
Ajax made full use of their numerical advantage in the 43rd minute when Grobler sneaked ahead of his marker at the near post to nudge home a cross which was delivered by Nhlanhla Vilakazi on the right flank.
Despite playing with one man less, Usuthu continued to see more ball possession in the second half, but they were guilty of giving the ball away in the final third.
Then in the 78th minute Ayanda Dlamini tried to combine with substitute Goodman Dlamini inside the penalty area, but the latter directed his low shot straight at Ajax goalkeeper Sander Westerveld.
Goodman Dlamini had another chance to level matters eight minutes later when the Ajax defence allowed the ball to bobble inside the penalty area, but he rushed his shot over the crossbar.