The three-time major champion posted a final round 66 to finish on 25 under, one stroke ahead of national Retief Goosen.
Goosen, winner of the tournament in 2005, finished strongly closing out his round with three successive birdies, but it wasn't enough to overhaul Els who had led since the halfway stage.
Earlier, Els had looked as though he may give up his lead after bogeying the par five eighth hole. But back-to-back birdies at the ninth and the 10th, followed by a birdie two at the 12th put the 41-year-old decisively back in control.
"This is a extraordinary day for me as I faced a lot of pressure, given I was up against great players and a great course," Els said.
"I realized from the start of the day that Retief was the one I had to finish ahead of. He has beaten me down the final straight quite a few times so it was immense to victory this time," he added.
