Rilee Rossouw was called up to South Africa's ODI squad as a replacement for the injured AB de Villiers, ahead of the match against Ireland at the end of last month. He ended up replacing an ill Hashim Amla at the start of the Australia series last week. To say that Rossouw had big boots to fill is not an understatement. But try two fifties in two matches on for size, and it seems Rossouw could be a perfect fit.

So, why the in-house handwringing from captain Faf du Plessis, who said he wanted Amla back in the XI but the selectors decided otherwise? "Hashim averages 52 in one-day cricket, so, for me, you can never replace that," du Plessis said. "And if you miss one game for the flu, then it's always the respect that you're given to come back into the team. So, if it was my decision, I would definitely bring him back, and the same with the coach, but the selectors felt the combination at the top was working really well."

Perhaps, the stand-in captain just wants protocol to take precedence, and it seems he is not the only one. Television and social media commentators were aghast at Amla's omission on Sunday, and there is every indication that he will be back for the third ODI. And where will that leave Rossouw, mentally and literally? "When we had our team culture camp, we spoke about our playing bubble. People can say what they want to say, as long as it doesn't affect how we are going about our proceedings and how we play," Rossouw said. "If Hashim comes back and I am playing, I will probably slot into the middle order."

Rossouw has batted at Nos. 4-7 in 18 of the 33 ODIs he has played, and come out at No. 3 eight times, so he is most familiar outside the top two but more successful in it. In the seven matches in which Rossouw has opened, he has averaged 39.57 - compared to an overall average of 35.53 - and scored one of his two hundreds and two of his seven fifties. He admitted he prefers getting to the crease as early as possible, and that the constant shifting up and down the order is challenging but possible with proper preparation.

"I don't mind anywhere in the top of the order. I prefer the new ball, while the Powerplay is still on so I can pierce the field and play good cricket shots," he said. "It's very difficult to change batting positions. It's about how you prepare for where you are going to bat. If your preparation is good enough for the scenario out there, then your execution should follow."

Rossouw was fairly pleased with his execution so far in this series, barring the failure to get to three figures. "I am quite disappointed in that. In the first game, the reverse sweep is my shot, so if I go out to that, it is hard lines, but it's my shot," he said. "I am more disappointed with the way I went out at the Wanderers. It was quite soft."

At Centurion, Rossouw was out lbw, attempting a reverse sweep off the first ball he faced from Adam Zampa. At the Wanderers, he miscued a pull shot, and though the selection of both strokes is questionable, Rossouw insisted the second one was worse because he played it when he was closer to his century. "I have been hard on myself for that. I pride myself in making the most runs that I can. So 75 is all right, it's good, but I am more interested in the three figures."

Without a hundred, Rossouw knows it will be more difficult to keep his place, especially in a line-up that should welcome Amla back at some stage, and with a crammed middle order. Du Plessis and JP Duminy have both found their touch, leaving little for David Miller and Farhaan Behardien to do, and Rossouw will have to displace one of them if he moves down. "It's a tough gig, because if you are not performing, there's a guy that can do the same job as you, if not better," he said. "When you put on that shirt, you want to perform and do your best because you know there's someone else who can take your spot."

At least South Africa have the problem of plenty, unlike their opposition, who now face three must-win matches if they are to win the series. Although Australia have appeared a little flat so far, Rossouw expects them to surge back, especially when they hear of South Africa's intention to close out the series with two games to spare.

"If they don't bounce back, then they don't deserve to be the best team in the world. I am sure they will come out fighting with big swings, and it's up to us how we will counter-punch," he said. "The guys had two serious performances and we are ready for the third game tomorrow, and we want to wrap it up. Australia are the best team in the world. If you give them a sniff, they come back hard. We want to finish this series tomorrow."

Firdose Moonda is ESPNcricinfo's South Africa correspondent