The South African, who won each of his four European Tour titles to date in 2012, made a scintillating start to this event on home soil, carding a round of 62 on the opening day at Leopard Creek Country Club.
And, while he was unable to match that 10-under round, only Francesco Molinari was able to outperform Grace on Friday as the 26-year-old moved to 16-under for the tournament.
Molinari now sits second after carding 65 on day two.
Grace picked up where he left off - hitting four birdies by the midway point after starting on the back nine.
Another gain on hole one was followed by Grace's first bogey of the tournament on the fifth, but two birdies on the final four holes saw him establish a comfortable lead.
And Grace says he feels comfortable on the course.
"This is a golf course that suits me and if I keep hitting everything in play the way I have been the first two days then it should be a good weekend," he said. "The pins have been kind so far; I'm sure there's going to be a couple of tricky and sticky pins out there on the weekend, but I'll just stick to the game plan and see what the next two rounds offer us.
"Today was great – the ball striking wasn't as good as yesterday.
"I kept it going - there was a little bit of a slip there due to a lack of concentration but it happens.
"I've been hitting it well and kept on hitting good putts out there."
An eagle on the 13th was the highlight for Molinari as he posted a round of seven under par, while a second successive 67 saw Tjaart van der Walt climb to third.
Jake Roos struggled to live up to the standard he set on day one as he followed up his seven-under 65 with a level-par round on Friday, while Danny Willett dropped one place to tied-fourth with a round of 69.