"It just happens," he said of the crucial moment.
"I thought the ball was going to roll a bit further. It didn't and I thought 'uh oh'," he said last night.
As it happened: Black Caps win nail-biter
"Sometimes you get a bit of luck; sometimes you get a bit of bad luck. You just think about the next ball and try to stick to your plan."
It was Williamson's eighth T20 half century, and his 82-run stand in nine overs with opener Martin Guptill (65 off 40 balls) was just the ticket New Zealand needed, having figured out there was no demons in a peculiar-looking pitch.
Read more: Chris Jordan's catch to dismiss Colin de Grandhomme could be the best you'll see
Debutants Mark Chapman (20 off 13 balls) and Tim Seifert (14 not out off six), both hit two sixes apiece and provided just the right extra push in the closing stages.
"They added a lot of energy, which was great," Williamson said.
"They came in with limited opportunities and the way they approached their innings and performed, they were outstanding. It was brilliant to see."
Read more: Kane Williamson stars as Black Caps edge out England
England gave themselves a decent chance of overhauling the record-equalling score on the ground by New Zealand, but 25 off the last over was too much.
Still, they showed their tour here promises to be highly entertaining.
Opener Alex Hales, lefthander Dawid (crrt) Malan and a spot of late-innings hitting from left arm seamer David Willey gave England a sniff. However Trent Boult – one of three New Zealand bowlers to grab two wickets – shut the door on the chase with two wickets in consecutive balls in the 18th over.
New Zealand got top value out of part time medium pacer, Colin Munro, who bowled two tidy overs for 11; while legspinner Ish Sodhi was clever, and pricy, by turn, finishing with two for 49 off his four overs; and left arm spinner Mitchell Santner was highly effective with two for 29.
Santner did blot hisnight by dropping one of two chances which went down during the chase, spilling a catch on the long on boundary off Sodhi, which rebounded over the rope for six.
Tim Southee, usually a safe pair of hands, then completely missed a skier at deep mid wicket off Santner.
The teams reconvene at Hamilton next Sunday by which time the result may be immaterial to the Tri-Series.
If New Zealand beat Australia at Eden Park on Friday night, the two will meet again in the final on the same ground next Wednesday.
England must hope for an Australian win, then beat New Zealand themselves in Hamilton.
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