Murless, who has held a piece of the lead now after all three rounds, completed 54 holes at 11-under-par 202, which is good for a two-shot advantage.
Former South African amateur star Tyrone Ferreira is making his professional debut a memorable one. He also posted a 69 on Saturday and shares second place with Pablo Larrazabal (70) and Sven Struver (66) at minus-9.
Play was split over both the East and West Courses at Royal Johannesburg and Kensington Golf Club over the first two rounds, but shifted to the East Course for the weekend.
Murless began the third round with a one-shot edge, but quickly extended the lead to two thanks to a birdie at the first. He parred his next six holes until a two-putt birdie at the par-5 eighth got him to 11 under par for the championship.
Murless made a mess of the 10th hole, but still gave himself a long birdie look from the fringe. His putt came up 12 feet short and the three-time winner on the Sunshine Tour, which co-sponsors the event with the European Tour, missed his par effort.
At the 11th, Murless appeared to be unraveling a bit. After the bogey at 10, he drove into the left rough and did not reach the putting surface with his second. Murless got up and down for par, then hit some wonderful shots.
Murless' tee ball at the par-3 12th landed 10 feet from the hole. His birdie putt stayed above ground, but at the next hole, his birdie putt from 5 feet out rolled into the bottom of the cup.
The birdie at 13 gave him a three-shot cushion, but that evaporated quickly.
Struver birdied his final two holes to get into the clubhouse at 9 under par. Ferreira drained a 15-foot eagle putt at the par-5 closing hole to get in at the same number, but it was a mistake by Murless that really opened the door.
At the par-4 17th, Murless found the fairway off the tee, but hit a terrible approach into a bunker on the left. He blasted out 10 feet past the flagstick and failed to convert his par try.
Murless only took a one-shot lead onto the 18th tee and pulled his drive into the left rough. He could not reach the green in two and hit his third to 12 feet. Murless holed the birdie putt to give him a two-shot cushion in his pursuit of a first European Tour victory.
'I hit everything nice and solid until about the turn,' acknowledged Murless, who was born and raised in Johannesburg. 'Then, for some reason, I started to struggle a little bit.
'I don't what happened on 17 when I missed the green from 90 meters. Maybe it was a little bit of adrenaline and it came out a bit hot and I had a bad lie in the bunker for my third. However after both my playing partners made birdie on the last green I had to follow them in for one of my own.'
Darren Clarke (67), Magnus Carlsson (69), Ross McGowan (64), Garth Mulroy (67), Paul Waring (69) and Chris Williams (67) are knotted in fifth place at 8-under-par 205.
Defending champion Ariel Canete carded his third consecutive round of 1-under 70 and is part of a large group tied for 35th place at 3-under-par 210.