The pair were tied at 14-under 274 after 72 holes at Pearl Valley Golf Estates, and Sterne (66) birdied the playoff hole after Maybin (70) pushed his tee shot into a fairway bunker and was only able to make par.
Sterne hit what he described as the best bunker shot of his life on his final regulation hole after pulling his approach left to land in the sand with very little green to aim for.
I dont even remember hitting that bunker shot, Sterne said. I was trying to hit it to 15 feet, to be honest, to give myself a chance there, and it just came out perfect. It came out to 3 feet, way better than I could have hoped.
Sterne, who won the Alfred Dunhill Championship last week to secure South Africas Sunshine Tour Order of Merit, birdied that hole to take the clubhouse lead.
Ernie Els of South Africa had set the early pace with a 64, while Lee Westwood of England was the third-round leader at 14 under.
No one was running away with it, although obviously Ernie made a big move, Sterne said. Going into the back nine, I thought if I could just get to 14 or 15 under, you never know what can happen coming down the stretch.