Williamson and Ronchi were punishing off the back foot and showed the sort of application that's going to be required for New Zealand to be considered a contender at the World Cup.
Equally, four players on the World Cup squad periphery galvanised New Zealand's hopes.
Dean Brownlie and Anton Devcich produced the first ODI century opening partnership for New Zealand since consecutive efforts from Martin Guptill and Rob Nicol against Zimbabwe in February 2012.
Earlier, New Zealand responded to a couple of Pakistani batting counterattacks to capitalise on an outstanding start gifted by Matt Henry and Mitchell McClenaghan.
The pair reduced Pakistan to 20 for three in the fifth over. Henry had Ahmed Shehzad caught behind and bowled Asad Shafiq; McClenaghan coaxed an edge off Younis Khan to slip Ross Taylor.
Pakistan made 252 from 48.3 overs.
Support also came from Adam Milne, particularly in his final three-over spell. He dismissed Shahid Afridi for 27 from 14 with a useful bouncer after getting hit for consecutive sixes.
Taylor played a pivotal fielding role with three catches to continue his extraordinary record. His 94 catches in 139 matches is at a ratio of 0.686 per innings. The next best catcher in ODI history - for those who have played 10 or more matches - is Australia's Mike Hussey at 0.570 per game.
Henry produced a revelatory performance with four for 45 in his third ODI. He got the ball to spit off a flat wicket, similar to that which was seen on the opening day of the third test. His dismissals were all top order players - Mohammed Hafeez for 76, Shehzad for a duck, Shafiq for one and Misbah-ul-Haq for 47.
The latter caught behind decision was reviewed with Misbah adamant it only touched his forearm. There was not enough evidence to suggest it missed his gloves. Henry barely appealed; wicketkeeper Ronchi was insistent. It broke the 64-run fifth-wicket partnership.
McClenaghan's aggression was palpable on his way to three for 56 in 9.3 overs and Milne's hunger came to the fore with two for 53. Anderson also put together a series of useful slower and shorter balls on his way to one for 35 from seven overs. He took the vital wicket of Sohail for 33 to break a fourth-wicket stand of 77.
The New Zealand bowlers delivered shorter lengths at Sohail who dominated with 85 in the first ODI. In fact, the tactic stifled the majority of Pakistani batsmen. The spinners struggled the most. Daniel Vettori took none for 48 and was subjected to two sixes down the ground, while Devcich got minimal momentum conceding 14 from two overs.
Williamson, who was cleared this week after a suspension for throwing, opted not to bowl himself.
He made some useful captaincy calls instead. Bringing Henry back for his second spell and setting a trap for Hafeez at deep mid-wicket worked, as did returning Taylor at slip for Wahab Riaz. He also helped restrict Sarfraz Ahmed's flow of runs to 23 from 31 balls by stacking the offside field.
Kyle Mills missed the match after suffering a groin strain at training yesterday. Martin Guptill continued to rehabilitate a hamstring injury.Cricket
