Grobler rounded off a clinical counter-attack to put a poor first half performance from his teammates to bed, lifting Matsatsantsa to 37 points and up to fifth position, trailing the Buccaneers only on goal-difference.
A busy start saw plenty of exchanges in the middle of the park, but little in the way of goalmouth action as both teams settled into their rhythms and structures.
Thuso Phala had a great chance saved on 20 minutes, after the fiery midfielder received a neat ball over the top, before dancing past a defender and getting his shot away, which was palmed away well by Pirates’ Senzo Meyiwa.
The visitors replied with a shot from former United playmaker Kermit Erasmus, before Tlou Segolela rattled the upright with a left-footed effort from a tight angle shortly before half-time.
Level at the break after a dull first-half affair, Bradley Grobler had a fine chance go a-begging when Phala’s delivery into the area was cleared away in the nick of time by a retreating Pirates’ backline.
Meyiwa was called into action again after the hour mark as he was forced to turn Enocent Mkhabela’s effort around the post for a corner.
The home side looked destined to share the spoils with their counterparts from Johannesburg, until Phala — for the umpteenth time on the day –delivered a pinpoint cross into the box from the right, leaving Grobler dead in front and with the goal at his mercy, as he slotted home emphatically for the late lead with under 15 minutes left to play.
Phala was in sublime form and continued to cause problems down the right flank, as he went agonisingly close in additional time to doubling United’s advantage, but pushed his effort inches wide of the mark.
The late charge was enough to keep the Sea Robbers at bay, closing out the narrow victory which condemned Pirates to a successive league defeat and bringing their fourth-place position under serious threat as the Premiership campaign approaches a thrilling end.