Clouds overhead, a green pitch underfoot, conditions were perfect for the kind of searing, prodigious late swing that Trent Boult specialises in, but the thing that really envenomed him - the thing that really set his spectacular burst of six wickets from 15 balls in motion - was rhythm.

Bowlers had nipped it off the seam all through the series, but Boult's spell was the first exhibition of vicious swing bowling - of movement in the air. He swung it exclusively left to right (in to the right-handers). Balls that seemed to be headed way past off stump veered dramatically toward the wickets halfway through their trajectory. Some of those he had trapped lbw were so convinced that the ball was sliding past the stumps, they offered no shot.

"It's all about rhythm - for me anyway," Boult said. "It's about getting into a groove. A couple of wickets gives you a bit of confidence and just lets you go about your thing. Sometimes you're not really thinking about much and it's kind of happening for you. The plans were simple this morning. It was about building pressure on the guys and it was nice to exploit a bit of their weaknesses with some swing bowling. I was just trying to pitch the ball up really."

Boult had been bowling excellent lines and lengths through the course of the series, with little to show for it before this morning. In Wellington, he had collected figures of 2 for 145 from the match, despite comfortably having maintained an economy rate of less than three. On the first day in this match, half of his 10 overs had been maidens, but he had still been unable to claim a wicket.

"It's a funny old game really - it's probably not how I expected it to happen this morning," he said. "I suppose it can be easy to get frustrated a little bit, trying to figure out why things aren't happening. I suppose with a little bit more experience - I'm getting old now - I'm realising it can be a funny game, and it's probably about turning up with the right attitude, and trying to put the ball in the right areas. I was just lucky enough to get a bit of reward."

It had been Boult's new-ball partner Tim Southee who had reaped "rewards" for the pressure the pair had built together on the first six days of the tour - Southee claiming a five-wicket haul on the first day in Wellington, and three wickets on day one at Hagley Oval. When Boult was getting wickets on day two, however, Southee did plenty to contribute. He took two catches at third slip, and also ensured that the in-form Angelo Mathews remained at the end he was bowling to, so that Boult could take aim at the Sri Lanka tailenders.

"It's one of the strengths of the side, and of the bowling unit, to bowl for each other," Boult said. "It's a strength to stick to that plan and be willing to hang in there and be patient. Timmy got the rewards last week and I'm sure there's just rewards for the other guys around the corner as well. I think that's one of the main reasons we are so successful is the way we bowl from both ends, and bowl in those partnerships."