Steven Smith started this series as a somewhat miffed 12th man in Perth.

Three games on, and in Canberra, Smith showed precisely why he felt that way: his sparkling 73 lifted Australia's tally beyond South Africa's reach, causing AB de Villiers to dub him "a captain's nightmare" and Aaron Finch to go even further by naming Smith in the same breath as de Villiers himself.

This was high praise indeed for a man who had been elbowed out of the XI by the return of Shane Watson, the fitness of Michael Clarke - regrettably an occurrence no longer guaranteed - and the selectors' faith in Glenn Maxwell and George Bailey.

Clarke's hamstring opened up a spot for Smith, and on a Manuka surface where batting became more difficult the older the ball became, his nimble feet, hands and mind stood out a mile, not least when his extraordinary "nutmeg" shot in the final over completed a dire day for Morne Morkel. Smith looks nobody's idea of a batsman surplus to Australia's World Cup team.

"I think his strength is that he's got a lot of energy at the wicket," said de Villiers. "He's almost a captain's nightmare when he comes to the crease after 25-30 overs, especially on this kind of ground where you know you're not going to get three and four wickets in patches.

"You're going to have to work hard for your wickets, and if you get a busy cricketer at the wicket it makes it very difficult for you to control the innings, to keep your rhythm and just to pace it a little bit better. He makes it really difficult for us in doing that."

Finch had played diligently and well to battle out of his own dalliance with poor batting touch, 109 a first score of better than 50 since September the result of some more circumspect batting than he is known for. But it was Smith who won man of the match, and rightfully so in Finch's opinion, for he knew how the softer ball became harder to bully in later overs.

"Smithy played one of the great innings I thought, the way he came in from ball one and struck it into the gaps and ran hard and never allowed the bowlers to settle," Finch said. "He was moving around the crease and played a couple through his legs, I don't know how he does that.

"He was super and that's really shown the class of the player. We've seen that in Test cricket for a while and in one-day cricket it's starting to get better and better and more consistent. The beauty of Steve is he's a great player of spin, so teams are reluctant to bowl that at the start of his innings because that does allow him to get away.

"You're going to have to work hard for your wickets, and if you get a busy cricketer at the wicket it makes it very difficult for you to control the innings, to keep your rhythm and just to pace it a little bit better. He makes it really difficult for us in doing that" AB de Villiers

"At the same time he bats in the middle order in Test cricket and can play pace. So he's a hard guy to tie down - I think he's a very similar type player to AB de Villiers, you really struggle to bowl dot balls to them in a row, and through the middle of the innings if you've got somebody who's constantly getting off strike or hitting boundaries it's such a hard thing to defend against."

Smith's upward trend in this series reflects his wider international career, which began with confusion over his precise role - batsman, an allrounder or leg spinner - then went through a period of domestic exile and solid learning, before blossoming into the player many thought he would become when he burst into the NSW team in 2009.

He has provided an example of how to grasp technical and mental maturity that others, notably Maxwell and Finch himself, would do well to remember. Finch's innings showed that he too is learning and evolving, on the sort of pace that will allow him to perform staunchly at the World Cup next year.

"I knew I'd been hitting the ball well lately but when you do get a couple of starts and you miss out a couple of times things start to play on your mind a little bit and you start to wonder how good a form you're actually in," Finch said. "But I still had confidence in my game."

As for the pressure for spots that has seen Smith running drinks, James Faulkner sent for Sheffield Shield duty and Maxwell cooling his heels on the Manuka boundary, Finch felt it a force for good, so long as the players kept thinking in terms of the team and each other.

"It's a real positive when you have guys challenging for spots because you know that's driving you to perform well, and if you don't perform you're sitting on the pine," he said. "Nobody's safe in the side except the skipper - when you have that competition in the side it's really quality.

"Guys are still helping each other out. There's none of this 'I'm trying to look after my own spot', it's just about helping the team win games and us becoming the best players we can."

Smith is getting closer to that peak than most.