Garcia's error-free round was in stark contrast to that of world number six McIlroy, whose level-par 72 was characterised by some wayward driving.
"I think if I can manage to shoot somewhere in the 60s, maybe 70 might be enough, " said Garcia, who opened the tournament with an 8-under 64 before carding 71 and then 71 again on Saturday.
"My first year on the European Tour gave me a lot of golf learning experiences, a lot of great finishes", he said about his year which will give him plenty of confidence coming to this week at Gary Player Country Club at Sun City.
"It felt great. I rolled the ball nicely, I made some good putts here and there".
At one stage Garcia led by five shots as he opened his round with birdies on the first two holes, but four bogeys, including the 18th, spoiled his day and allowed others to close in around him. It's not ideal to see three-putts, but some of the pins were nearly impossible to get to and when you leave yourself 20 or 25 feet putting over hills then it is going to happen sometimes. The golf course is a ball-striker's paradise, really. I would certainly have taken that on Thursday morning.
Oosthuizen recovered, though, with a superb approach to the 10th setting up an eagle, followed by back-to-back birdies on 14 and 15.
In addition to Oosthuizen, Dylan Frittelli is also leading the South African charge as he also shot a 67 on Friday to leap to four-under-par overall, five shots behind Garcia. There's a few I feel like I could have challenged a bit more for.
He whispered to Player as he handed the crystal trophy back to the tournament host in a televised ceremony on Tuesday: "Hopefully we can keep it in South Africa again this week".
"If I can tidy that up tomorrow I should be alright, but obviously I'm pretty disappointed right now", said McIlroy. "Hopefully I can get off to a good start this week". "It's not ideal to see three-putts, but some of the pins were nearly impossible to get to and when you leave yourself 20 or 25 feet putting over hills then it's gonna happen sometimes", a phlegmatic Garcia said after his round on Friday. "I think I played very smart, I probably wasn't swinging unbelievably but I kept it under control and kept the ball in play all the time which is hard to do here".