On a sticky Edgbaston pitch, only three players managed to score at more than a run-a-ball. A series of off-cutters and cross-seam deliveries made scoring challenging but Williamson held his nerve with an innings that lasted double the length of time than anyone else.
Supported by the fluent Colin de Grandhomme, who smashed 60 from 47 and conceded just 33 runs in ten overs with the ball, a 91-run sixth wicket stand proved pivotal to the outcome of a traditionally close fixture.
Under humid, overcast conditions, the bowling on offer was exquisite, underpinned by three deliveries that took out the stumps of the South African top order within the first 28 overs.
Quinton de Kock was beaten for pace by Trent Boult, Faf du Plessis deceived by the impressive Lockie Ferguson and Hashim Amla spun out by Mitchell Santner; the latter proving critical with the opener starting to spread his arms on 55 from 83 deliveries as the run rate nestled at just four per over.
With New Zealand picking up wickets at regular intervals, only Rassie van der Dussen managed to find his rhythm at any sort of pace. Three sixes took him to 67 from 64 balls as South Africa stumbled to 241-6, seemingly 20 or 30 runs short of a par score.
Williamson and de Grandhomme strike
On a pitch clearly effected by the morning weather conditions, which led to a delayed start and one over per side scratched off each innings, the Kiwis also struggled to find their form. Kagiso Rabada dismissed Colin Munro in the third over with a caught and bowled effort, a regular feature of this World Cup.
Martin Guptill and Williamson rebuilt with true New Zealand grit but the former's hit wicket dismissal at 72-2 started a mini collapse - Chris Morris picking up wickets in successive overs as the Kiwis crumbled to 80-4.
Jimmy Neesham scored an important 23 but when Morris also picked up his wicket, the contest looked to be swerving in favour of South Africa.
Yet it was the introduction of de Grandhomme, on a pitch that he has a strike rate of 170 for Warwickshire in Twenty20 cricket, that maneuvered the pendulum oncemore. In a counter-attacking style, contrasting to Williamson's composed drop and run, the duo worked their way to within 14 of the target with just two overs remaining. Albeit, after a series of dropped catches, missed run outs and a decision not to review a slight edge through to de Kock behind the stumps.
However, for all of his fine work with bat and ball, de Grandhomme imploded when he played a needless upward shot straight to du Plessis when the asking rate was little over a run a ball.
But it was two majestic Williamson shots, synonymous of his three and a half hours at the crease, that decided the outcome. With Lungi Ngidi winding up the pressure as the the target became 12 from 7, New Zealand's captain played a deft touch that ran down to the third man boundary.
There was still plenty of work to do but when he returned to the strike with 7 required from 5, Williamson judged the plan of Andile Phehlukwayo to perfection. Rocking back into he crease, he pulled a short and slow delivery over mid-wicket for his first maximum in 137 deliveries, bringing up his century in the process.
Even when he stroked the winning shot a ball later, celebrations were minimal as Williamson, ever the professional, looked as if he was planning on batting into the night for a double-century.