AmaZulu received an early chance when Marc Van Heerden fired in a free-kick over the top from the edge of the box.
After a slow opening with no goalkeeper tested, Bucs threatened with a shot at the back post from Augustine Mulenga reducing Siyabonga Mbatha to save from point-blank range.
With minutes to halftime, the Sea Robbers deservedly took the lead after Thembinkosi Lorch pounced on poor defending for an easy poke to an empty net.
The lead lasted only a few minutes as Bonginkosi Ntuli came up at the other end to found the equaliser with a low header from AmaZulu’s first effort on target.
Pirates tried to go into the break in front but were undone by poor finishing from Vincent Pule at the end.
The second half started the way it ended, with both sides having a go at each other while Vincent Pule missed from five metres after founding himself in a perfect goal-scoring situation.
Argentine Emiliano Tade, who set-up Ntuli’s equaliser missed target by a whisker following a shot metres from outside the area.
A darting run from Pule out right saw him pull a cut back before taking a curling shot that dipped off the ground, forcing Mbatha to a punch away after being nearly caught off guard.
After a few half-chances and near misses from both sides, Tade missed an absolute sitter from a long ball missing the net by just a few centre-metres 20 minutes to the end.
Pirates missed a perfect opportunity to seal the match in the final stages when Lorch brought down a long ball to tee-off to Justin Shonga who failed to capitalise due to a poor first touch.
A minute to the end with extra time looming, Usuthu nearly stole the victory when Tade combined with Ntuli and found the former at the end of the shot but good defending kept him out.
Out of nothing, Bucs regained the lead with a snap shot from Xola Mlambo from outside the box after a pass from Thabo Qalinge a minute into extra time.
After Usuthu plotting a come back, Bucs went into the next phase in the lead with both sets of players tiring.
In the second phase, Lorch completed his brace when he finished off a pass from Qalinge at the near post.
Lorch was in it again a minute to fulltime when he beat the offside trap to get on a long pass but was denied by the post.
Usuthu finished off a man down after Samuel Darpoh was handed his marching orders for a reckless challenge as Bucs advance to the last four.
AmaZulu: Mbatha, Sikhakhane, Mabiliso, Ouro-Akoriko, Gumede, Darpoh, Vilakazi, Van Heerden, Sithebe, Tade, Ntuli.
Subs: Murambadoro, Gumede, Morton, Ncube, Maduna, Cele, Khenyeza.
Pirates: Mpontshane, Jele, Munetsi, Mbekile, Maela, Motshwari, Mlambo, Pule, Lorch, Shonga, Mulenga.
Subs: Mabokgwane, Da Silva, Makola, Mako, Mntambo, Qalinge, Kutumela.