Trent Boult bristled when he was reminded of the back niggle that kept him out for a part of last year's international action.

"What injury?" was his response, when asked if he considered himself to be back at his best. Then, he brushed it off as something confined to ancient history, not a mere 12 months ago, and insisted it had no impact on the speeds he was bowling.

In Zimbabwe, Boult averaged in the late 120s. Perhaps what he could not say was that the surfaces and the opposition - unresponsive and inexperienced - did not require him to bend his back. Perhaps he knew he needed to save that for South Africa. And he has.

Be it the natural progression that comes with recovery, or a conscious effort to be more clinical, Boult was just as difficult to get away and more dangerous in Durban. His first two spells cost just 18 runs and yielded two wickets, both off good deliveries and not poor shots, and it was only in the third spell, when he searched for reverse swing that things became a little untidy.

More notable was the fact that he was quicker than he was in Bulawayo - with an average speed of 134 kph and hitting 141.5 kph for the day's fastest ball - and he continued to find late movement, forcing the batsmen to pay full attention to every ball. "Trent bowled really well to start, and that set the day up for us. He looked like he was in great rhythm," Neil Wagner said.

Boult got rid of two of South Africa's most assured batsmen - opener Stephen Cook and Hashim Amla - and bored their way into a still-fragile middle-order, which could not ride out pressure for long enough to post substantial scores. Fortunately for New Zealand's other bowlers, they did not need to emulate Boult to get reward. "Quite a few of us got out to deliveries in ways that could have been avoided," Amla said.

Dean Elgar, JP Duminy and Quinton de Kock were the guiltiest parties. All three squandered starts with careless strokes, which seemed to be in line with South Africa's policy of starting more strongly than they have in the past. Instead of criticising it, Amla examined the merits of the more aggressive approach, while adding that South Africa would need it to pay off before they can completely embrace it.

"I tried to bat normally. It wasn't a conscious effort to score quickly. If I look at anybody else, they were hitting the half-volleys and cut shots. That's what you need to do to score runs," Amla said. "Someone like Quinny [Quinton de Kock] has been around for three or four years and he is aggressive by nature. He plays that way. He got a quick 30 and had he not got out, we would have been in a good position. As he becomes more experienced, he will work it out. He played exceptionally well to get us some momentum, but, unfortunately, he didn't bat through."

Now South Africa find themselves in what Amla has admitted is "not a great position," as they look to "scrape some runs tomorrow morning," against a New Zealand attack that can see the finish line and understands that it needs to approach it as Boult did on day one. "With the wicket having a little bit more bounce here, your margin of error was a little smaller in Bulawayo," Wagner said. "As a bowler, you can get a little bit carried away here, but I thought everybody bowled exceptionally well in partnerships today. We are pretty happy with where we are, but we know we still need to get two crucial wickets tomorrow and then go in with the bat and apply ourselves."