Williamson and Elliott, who reassured with 44 off 58 balls, delivered 88 runs for the fourth wicket in 17 overs. Corey Anderson cranked the momentum with 47 off 44. The New Zealand batsmen benefited from their bowlers' performance at the death. Mitchell McClenaghan, Tim Southee and Anderson conceded just 65 runs in the last 10 overs and took six wickets.
"We started a bit slow with the ball but it was great to take wickets at the back end to restrict them to under 300," Williamson said. "It was always going to be a tough chase so it was a good hit out for our batsmen to build those partnerships and learn how to take down a bigger total. It was about constantly reading the situation and doing what is required."
Sri Lanka's twin 37-year-olds Kumar Sangakkara and Mahela Jayawardene treated fans to a symphony of stroke-making after the visitors won the toss and batted.
The customary elegance of Sangakkara (76 from 83 balls) and Jayawardene (94 off 82) showed why they're a World Cup threat. The pair oscillated between playing the ball along the ground and hitting in calculated fashion through the air. Milne spent some time off the field during the innings with sore ribs but put together a tidy display with two for 28 from seven overs. Coach Mike Hesson said Milne's injury will be reassessed tomorrow.
Daniel Vettori fulfilled his usual containment role, taking no wickets but conceding just 4.1 runs-per-over through the middle overs. Brendon McCullum produced highlights in the field. His chase-and-dive routine to save a boundary resulted in him smashing into a hoarding and the crowd erupted. His catch of Thisara Perera, running the length of a pitch was a gem. He also completed a final-over run out of Nuwan Kulasekara after spilling what would have been a catch-of-the-summer candidate 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.
Ronchi pulls out all stops in power-packed 48th
If the fourth one-day international was an arm wrestle, Luke Ronchi channelled Sylvester Stallone in Over The Top.
In the 48th over, with 25 runs required off 18 balls, Sri Lanka had powered back into the match. Ronchi wasn't having any of it. He pinned the Sri Lankans with a scorebook entry that read 4, 6, 2, 0, 6, 6.
All of the boundaries went in the V and the sight screen doubled as a trampoline while Ronchi dealt to Sri Lankan medium-pacer Thisara Perera. Perera struggled to adjust to Ronchi's power and temerity at such a pivotal point, finishing with one for 58 from eight overs.
The wicketkeeper-batsman, or should that be batsman-wicketkeeper, underlined why he has been selected as a premium closer. His ability to move into position with exceptional bat speed, backed by his dashing between the wickets, make him valuable.
Ronchi finished on 32 from 15 balls and gave Daniel Vettori the luxury of stroking the winning runs through cover from the first ball of the 49th over.
Kane Williamson, the architect of the win with 103 from 107 balls, said it allayed any anxiety in the dressing room.
"There was so much cricket to go with three overs left, but the way 'Ronch' played was outstanding. He took all the pressure off. He's done it a few times now and hopefully we'll see a bit more of that. "
Sri Lankan spinner Sachithra Senanayake also acknowledged the innings' impact, especially after Perera had taken one for 10, including the wicket of Williamson, from his previous two overs.
"Thisara bowled well but unfortunately in that last over he couldn't match Ronchi who smashed it."
Since the start of the season Ronchi has made 343 runs in nine innings at an average of 42.87, and, more importantly, a strike rate of 110. He has been dismissed for less than 16 once when he was run out without facing a ball in Hamilton last week.
The former Australian representative's performances have shown a trend which has transformed him from "potential talent" to "vital cog" in how New Zealand plan their World Cup.
