Grace had opened up a five-shot advantage at Leopard Creek Country Club with opening rounds of 62 and 66.
However, the South African could only muster a level-par 72 in his third round and will begin the final day just one clear of Lucas Bjerregaard, who reached 15 under with a 66.
Bjerregaard enjoyed a superb finish, following up an eagle at 13 with birdies at each of the last three holes.
That lifted him above Danny Willett, whose 67 moved him to 14 under, while Francesco Molinari went round in 70 to sit three behind Grace at 13 under.
Having dropped just one shot across his first two rounds, Grace was hampered by four bogeys on Saturday.
Initial stumbles at the second and sixth holes sandwiched birdies at the third and fourth, before Grace twice moved to 17 under only to then drop back to his overnight score.
Nevertheless, the leader remains confident he can get the job done.
"A couple of the easy holes I managed to make mistakes on," said Grace.
"But I’m in the position I want to be in and looking forward to tomorrow [Sunday].
"If you look at the positives, I'm still in good form and I made a couple of nice putts on the front nine. I'm one ahead and I'm raring to go tomorrow.
"I've only had good experiences going into a final round and leading. I'm looking forward to taking in those positive vibes."
Bjerregaard, the world number 273, also made two bogeys on the front nine, but picked up six shots on the back nine to surge into contention for a maiden European Tour title.
Willett, the champion at last week's Nedbank Golf Challenge, could yet triumph for a second week in succession after countering a double bogey at the ninth with seven birdies in his third round.
Charl Schwartzel's hopes of winning this tournament for a third straight year are over following a disappointing 76, which left the 2011 Masters champion 15 shots off the pace.