Torn is the prevailing state, with a nagging sense of doubt.
Williamson and Colin de Grandhomme's 91-run sixth wicket partnership rescued the Black Caps from a knife-edge predicament following another batting collapse similar to their narrow escape against Bangladesh earlier in the tournament.
In the space of four overs and eight runs, the Black Caps lost Martin Guptill, Ross Taylor and Tom Latham to crumble from a position of strength to teetering at 80-4 in search of a modest 242 on a difficult, sticky Birmingham pitch.
De Grandhomme strode to the crease and backed up his 1-33 by taking the pressure off with lusty blows and a brilliant half century; a display of mental fortitude under extreme pressure, only to hole out.
Kane Williamson of New Zealand and Faf Du Plessis of South Africa shake hands at the end of the match. Photo / Getty
Of course, it was again left to Williamson, ever the man for a crisis.
Without him, the Black Caps were shot.
This was not a knock of fluency. With typical composed, wristy accumulation Williamson steadied the ship; first with Jimmy Neesham, then de Grandhomme.
In between times the skipper struggled for timing and grappled with frustrations. Williamson took multiple blows to the body. He twisted the bat in his hands; shook his head, strode down and tapped the wicket, all in efforts to shake off the last ball and move on to the next.
It was anything but straightforward – as his unbeaten 106 from 138 balls attests.
De Grandhomme and Williamson both survived sharp chances in Imran Tahir's final over. There was a near run out with Williamson on 77, and he was also dropped off a no ball.
Once again, though, grit prevailed.
Williamson's late cut with the Black Caps needing 12 runs from seven balls was a thing of beauty. And then the six, the glorious six over midwicket, to notch his 12th ODI century and secure the win epitomised his ice veins.
With the Black Caps, though, it's still the classic question of when to commit to a relationship. Protecting feelings is a natural instinct until you know for sure trust exists between both parties.
With what's to come, that can't be granted yet.
Sure the Black Caps can only play who is in-front of them. Dispatching Sri Lanka and Afghanistan – and scraping past Bangladesh – is heartening, to a degree.
But with a divide emerging at this tournament, New Zealand are yet to face any of the three elite sides – India, England and Australia.
For that reason and that reason alone, diving off the deep end is premature. It's not water wings at the shallow end of the pool territory – more a case of still testing the waters.
Edgbaston revealed vulnerabilities as much as it did keep the Black Caps on a path to the knockouts.
In much the same context as Sri Lanka are no longer a force so, too, had the Proteas hit a low ebb prior to this match. Confidence had been eroded after one, lone win over Afghanistan. South Africa's semifinal hopes are now gone, too.
The absences of Dale Steyn and former skipper AB de Villiers, so often the inspiration, looked telling. And yet after another brilliant bowling performance, the Black Caps again invited the opposition back.
When Lockie Ferguson charges in to deliver fireball yorkers, those which evoke Shane Bond memories, and gives South African skipper Faf du Plessis a proper sendoff, the pendulum swings towards optimism.
With his aggressive, attack-the-body approach yielding 11 wickets in this tournament, Ferguson is fast becoming the lethal weapon of the Black Caps' attack.
Yet deficiencies elsewhere spark caution.
By New Zealand's standards their fielding was off – missing four run out chances and dropping two catches they would expect to take; Trent Boult and Ferguson the guilty parties.
While Matt Henry was near impossible to clobber, and Mitchell Santner produced a peach which gripped and spun to dismiss the well set Hashim Amla, Boult struggled for rhythm after his early strike.
Colin Munro also continues his unconvincing ways at the top of the order.
Nothing is a given with this team. But they sure promise a heart-jerking ride from here.
