The South African, who won each of his four European Tour titles to date in 2012, carded 10 birdies in a blemish-free opening round at Leopard Creek Country Club.
That was enough to lift the 26-year-old three shots clear of his nearest rival, compatriot Jake Roos, with Danny Willett - last week's winner at the Nedbank Golf Challenge - and Matt Ford one further back at six under.
Charl Schwartzel, who is seeking a third successive triumph at this event, began the defence of his title with a three-under 69, which was matched by Louis Oosthuizen.
Yet it was Grace, the world number 117, who stole the show in the European Tour's final event of 2014.
"I hit the ball superbly from the start to the finish,” said Grace.
"I can't remember the last time I hit 18 greens in a row, which is pretty nice, especially around this place where there are some daunting approaches.
"It's a good feeling going into the rest of the tournament."
Roos enjoyed a dream start to his first event as a full Tour member, holing a five-iron for an eagle two at the 10th, his first hole.
Six birdies followed for Roos, whose only dropped shot came at the 12th.
Ford, another rookie, bogeyed his opening hole but recovered superbly with the aid of eight birdies, while Willett matched Grace in keeping a bogey off his card.