Mattiace, who shared the first-round lead with Michael Jonzon, completed two rounds at 11-under-par 133. He was joined atop the leaderboard by Robert Rock (67) and Oskar Henningsson (64).
Richard Sterne is alone in fourth place at minus-10 after posting a 66 in the second round.
Mattiace, a two-time winner on the PGA Tour, was steady throughout his round. He birdied the par-5 second for the second day in a row. Other than that, he parred nine of the first 10 holes at Leopard Creek Country Club.
On the back nine, the American birdied 11 and 13 to move within one of Rock and Henningsson. Mattiace birdied the 15th for the second day in a row to match the other two at 11 under. He closed with three straight pars.
Rock, like he did on Thursday, did all of his damage over the last six holes. He birdied the second, but gave that shot back with a bogey on No. 5, then parred seven in a row from the sixth.
The Englishman birdied three in a row from the 13th to jump to minus-9. After a par on the 16th, Rock birdied 17 and 18 to share the second-round lead.
'I didn't really putt that well,' Rock admitted. 'The back nine was similar to (Thursday) really. The front nine is tougher and I didn't particularly play that well. But the driving has been the key so far really and that's left me in a good position on every hole.'
Henningsson played the back nine first and posted birdies on 11 and 15. He reeled off four straight birdies from the 17th to move to 9 under. Henningsson birdied the fifth and sixth to grab a share of the lead, then parred his final three holes.
Charl Schwartzel, Marc Cayeux and Tyrone Mordt all fired 5-under 67s Friday and are tied for fifth at 9-under-par 135. They were joined there by John E. Morgan (68) and Johan Edfors (69). David Lynn, Lee Westwood, Warren Abery and Jaco Van Zyl are one shot back at minus-8.
Jonzon posted an even-par 72 and shares 13th place at 7-under-par 137 with 10 other players.
The cut line fell at 2-under-par 142 with 70 players moving on to the weekend.