In injury time, Eleazar Rodgers created space for Isaacs and he made no mistake from close-range, but it has to be said that it was a goal against the run of play and nullified United's domination in terms of territory and opportunities.
The hot humid afternoon did the players no favours but they were determined to give the packed grandstand quality entertainment, although the spark was missing from this encounter.
In the fourth, Graham King's cross from the left was cleared away for a corner by Maritzburg central defender Mor Diouf to avert the lurking Santos forwards.
Byrone Hendricks put Musa Nyatama clear but the alert Santos keeper Lee Langeveldt time his intervention from the goalline perfectly to smother the chance.
Midway through the first half a stunning United move up the left channel that involved skipper Fadlu Davids twice and the talented Kurt Lentjies the same number of times ended with the inform Namibian Rudolf Bester forcing a full-stretch save from Langeveldt.
But Santos were not out of the picture entirely and they gave the United defence some tight moments, Jurie Basie forcing Elias Ngwepe to turn his shot away for a corner and their best chance – an Erwin Isaacs shot from the just inside the six-yard box that induces a stunning reflex save from Walters.
After the goalless first half, a jinking Felix Obada run from the right wing made space for sub Brice Aka, on for Bester, but his shot from 25 metres curled wide of the far post. The diminutive Obada was giving Ricardo Skippers endless headaches and the tide was definitely with the Team of Choice.
In the 55th Musa Nyatama's speculative effort from 35 metres sailed a good five feet over the crossbar, then Diouf got in the act but was off-target.
In the 58th Isaacs countered with a cross that Phikolethu Spelman steered goal wards but Walters was there to block the effort. Then at the other end almost immediately afterwards, Aka was troubling Langeveldt but needed to get his compass sorted out.
Play swung from end to end but the finishing was poor although Obada, in great form, again caused Santos a heart-stopping moment or two and looked the player hungriest to score in a lukewarm game.
However, in the 74th David collided with Langeveldt in a situation in the six-yard box where the goalkeeper's intervention prevented a near-certain goal.