Joint overnight leader Porteous made just five bogeys all week and hit every green in regulation at the Royal Johannesburg and Kensington Golf Club during his final round, while the chasing pack behind him struggled for the same consistency.
The 21-year-old finished the tournament on 18-under-par, two shots ahead of compatriot Zandersaf Lombard (71).
A further shot back was group of four players, all of who looked as though they may steal the limelight on Sunday before fading -- Swede Bjorn Akesson (68), England's Anthony Wall (72), Daniel Im (70) from the U.S. and home hope Justin Walters (69).
The three top-ranked players have also earned their place at he 145th British Open at Royal Troon in July -- Porteous, Lombard and Wall, who qualifies ahead of the other players on 15-under-par by virtue of his world ranking.
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(Reporting by Nick Said; editing by Martyn Herman)
