He eased 41 of his runs behind square using the pace of the ball without forcing his strokes.
New Zealand's World Cup-winning prospects now look skewed on the basis of his presence. New Zealand were lucky to have him after speculation he could be rested with his minor shoulder injury.
In 14 ODIs since the start of last year, he has passed 50 on nine occasions and New Zealand have not lost (eight wins and one tie). In the five other results there was a win (Williamson made 47) and four losses.
Williamson and Elliott, with a reassuring 44 off 58 balls obliged to deliver a partnership of 88 for the fourth wicket. Between the 16th and 29th overs they scored 66 runs off 84 balls yet hit just two fours.
Corey Anderson reinforced the momentum with 47 off 44.
The New Zealand batsmen benefited from their bowlers' performance at the death.
Mitchell McClenaghan, Tim Southee and Anderson concede just 65 runs in the last ten overs and took six wickets. That extended to eight wickets for 96 in the last 16.2 overs.
Earlier, Sri Lankan captain Angelo Mathews won the toss which treated fans to a symphony of stroke-making from twin 37-year-olds Kumar Sangakkara and Mahela Jayawardene.
The customary elegance of Sangakkara (76 from 83 balls) and Jayawardene (94 off 82) showed why Sri Lanka are a threat in the opening game of the World Cup and beyond in the tournament.
To put their consistency into context, it was Sangakkara's 92nd half-century in 370 innings and Jayawardene's 77th half-century in 409.
The pair oscillated between playing the ball along the ground and hitting in calculated fashion through the air. One stroke off Adam Milne was illustrative: New Zealand's fastest bowler delivered an effective short of a length ball and Jayawardene rocked back to late cut. The ball fizzed and swerved towards the third man boundary. Only some quality fielding from Grant Elliott saved an effortless four. Milne's pace was defused.
Milne spent some time off the field during the innings, apparently with sore ribs, but put together a tidy display with two for 28 from seven overs, enhanced by a wise review of an Angelo Mathews nick down the legside first ball to restrict Sri Lanka to 180 for four.
Milne complimented Southee's strong return of three for 59 from his allotment.
McClenaghan may have bowled too short early in the search for aggression, and dropped Sangakkara at short fine leg on 53 as New Zealand sought to reinforce the wicket of Tillakaratne Dilshan for 44 four balls earlier. McClenaghan resurged in his final overs to finish with two for 58, from 9.3 overs.
Vettori fulfilled his usual containment role, taking no wickets but conceding just 4.1 runs-per-over through the middle overs.
Brendon McCullum produced the innings highlights in the field. His chase-and-dive routine to save a boundary resulted in him smashing into an advertising hoarding with such an impact it echoed. The crowd erupted. Likewise his catch of Thisara Perera, running the length of a pitch to wide mid-off was a gem. He completed his display in the final over with the run out of Nuwan Kulasekara after spilling what would have been a candidate for catch-of-the-summer with a goal-keeper dive at mid-off.
Kulasekara couldn't be blamed for being distracted by the athleticism.
The fifth match is in Dunedin on Friday.
SCOREBOARD
