The duo put on an unbroken stand of 171 – surpassing the previous best mark set by South Africa’s Graeme Smith and Loots Bosman – as they easily chased down Pakistan’s total of 168-7 at Seddon Park.
Guptill was 87 not out from 58 deliveries, while Williamson, who was dropped on 61, struck a personal best of 72.
Umar Akmal made an unbeaten 56 off just 27 balls after Pakistan had won the toss and opted to bat first.
He hit four sixes and four fours as his fifty came up off only 22 balls, while fellow middle-order batsman Shoaib Malik contributed 39 off 30 balls.
Pakistan won the opening match of the three-game series by 16 runs on Friday night, with the decider to be played in Wellington on Friday.