Chasing 159 to win in Port Elizabeth, Australia looked set for victory when David Warner (67no) and Mitch Marsh narrowed the equation to 25 off 18 balls.

But Warner found himself stranded at the non-striker's end as Marsh, Matthew Wade and Ashton Agar came and went, and the Proteas prevailed by 12 runs to level the series.

The decider will be played in Cape Town on Wednesday night (Thursday morning AEDT).

Australia had earlier gambled on promoting Alex Carey to No.4, backing his batting against spin with crafty left-arm tweaker Tabraiz Shamsi controlling the tempo.

Carey was bowled by Lungi Ngidi for 14 and neither Marsh nor Wade, who followed him, made it past single figures.

Assistant coach Andrew McDonald said he expected captain Aaron Finch and coach Justin Langer to continue to experiment with the middle order.

"We've seen back in the Australian summer, I wasn't around, but we saw Steve Smith move down the order after a good start and Glenn Maxwell was promoted," McDonald said.

"Justin wants the players to be flexible. We're training them to be flexible and most of the time these guys have batted in all different situations and positions in the order.

"We'll be flexible between four, five and six and I don't think that'd be any different in Cape Town depending on the situation."

The breathless finish at St George's Park came after the Proteas were humiliated in Johannesburg in the first match of the series, bowled out for just 89 in a record 107-run defeat.

Quinton de Kock (70 off 47 balls) was the hero with the bat in Port Elizabeth but it was in the field where South Africa were truly redeemed.

Faf du Plessis and David Miller combined for an outrageous relay catch on the boundary to see off Marsh, while imposing pace trio Ngidi (3-41), Anrich Nortje and Kagiso Rabada all delivered inspired performances in the final overs.

"Sitting back there now, I think the players will feel it's a missed opportunity to win the series away from home, which is always a huge accomplishment," McDonald said.

"Now we've got our work cut out, a short turnaround to Cape Town and prepare as best we can to perform well there."

Warner and Smith (29) combined for a 50-partnership before Du Plessis snared a superb low catch at long on to dismiss Smith off the bowling of Dwaine Pretorius.

Smith had earlier delivered a moment of magic in the field.

Proteas skipper De Kock had already cleared the rope three times when he thumped an Adam Zampa delivery over deep midwicket, only for Smith to launch himself over the rope and throw the ball back before landing to save the six.

Smith almost did it again to dismiss Miller off the final ball of the innings but spilled the juggling effort when he tripped over the boundary rope.

Australian Associated Press