Garcia, chasing a second win in as many starts on the European Tour, followed up his opening-round 64 with a one-under 71 in Sun City to stay one ahead of nearest challenger Oosthuizen.
"You have to play the wind as you feel it because it is so inconsistent".
"It felt great. I rolled the ball nicely, I made some good putts here and there".
"It's going to be tough, there's no doubt about that", Garcia said.
"I'm going to need some great golf over the next two weeks, but we'll see what happens this week. When you can do that here, it's nice to be able to shoot a score like this".
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. "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.
Fox mixed three birdies, and an eagle - on the par five 10th - with three bogeys, and is tied with South African Branden Grace.
But after birdieing the par-four 13th, the four-time major victor twice drove into the bushes, leading to sixes at the par-five 14th and the par-four 17th.
"I'm pretty happy with the round", Oosthuizen told reporters. The driver let me down a couple of times which left me out of position and led to a couple of big numbers.
He was by no means the only player to struggle on the 15th though as 2011 Masters champion Charl Schwartzel (78) needed nine shots to complete the hole.
"It's a tournament I've always wanted to play as a kid, and I came here plenty of times with the family to watch", van Rooyen said.
McIlroy, ranked sixth in the world, said: "I need to tidy up my game off the tee".
"So I feel confident, but transferring that to playing well is a different thing".
"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".
"It was very hard this morning, the wind was blowing a lot and the first few holes were straight into it". "I still could have got away with a bogey, but both my caddie and I completely misread the putt".
- California wildfires force hundreds of thousands to evacuate, claim lives
- Backstage at Victoria's Secret Fashion Show 2018 - Best Photos Revealed!
- California wildfires: 9 dead, thousands displaced; celebrity homes go up in flames
- USA and China to Hold Security Talks Amid Trade Tensions
- WVU Ninth In Latest CFP Ranking - WVU | West Virginia Mountaineers sports coverage
- Delhi bans trucks as megacity chokes
- Don't over-do England/NZ Tests: Hansen
- United will go toe-to-toe with City, says Mourinho
- North Korea talks could affect U.S. military posture on Korean peninsula: Dunford
- Final Fantasy XV DLC cancelled as director quits