The 22 year old South African picked up five birdies and an eagle at Royal Johannesburg & Kensington Golf Club as he began the tournament on the West Course.
The field were split between the East and West courses for the first two rounds, before playing the weekend on the more difficult East Course.
And at eight under par Grace set a challenging early benchmark, racing three shots clear of England's Paul Waring and Dutchman Floris de Vries.
They were also playing the West Course, and had three and six holes left to play respectively.
Finland's Roope Kakko was the leading player on the East Course as he reached the turn four under par, to sit alongside four players from the West Course - Alexander Noren, James Kamte, Joakim Haeggman and Stuart Manley.