Mattiace was joined by England's Robert Rock (67) and Oskar Henningsson (64) of Sweden at 11-under 133.
Mattiace closed with three straight pars after four birdies in a bogey-free outing at the Leopard Creek in Mpumalanga, South Africa.
At 7-under 65, he shared the first-round lead with Sweden's Michael Jonzon.
Local favorite Richard Sterne is alone in fourth place at 10-under 134 after a 6-under 66.
Sterne, starting on the back nine, made a triple bogey eight at the 18th, but recovered with three birdies on the front nine.
"I had a little hiccup, but it was all pretty good, pretty decent," Sterne said. "I was never really in trouble."
The cut line was 2-under 142, and among the 70 players missing it were Mark Foster (143) and Rory McIlroy (143).