A masterful chase from Pakistan helped them claim a six-wicket win over the Blackcaps in Abu Dhabi on Saturday (NZ time).
After being set 154 for victory, Pakistan chased down the total with two balls to spare at Sheikh Zayed Stadium.
It meant captain Sarfraz Ahmed's Pakistan completed a remarkable 11th straight T20 series victory at 2-0 with one to play on Monday.
Opening batsman Colin Munro hit a blistering 44 off 28 balls, and Corey Anderson produced a brisk ending with an unbeaten 44 off 25 deliveries as the Blackcaps finished on 153/7.
Young left-arm fast bowler Shaheen Afridi grabbed 3/20 that included the wickets of Williamson, Glenn Phillips and wicketkeeper Tim Seifert.
The Blackcaps then put up a good fight with the ball and they were within a chance when Pakistan needed 40 from the last four overs.
But Mohammad Hafeez hit two sixes off Ish Sodhi's final over put the hosts in the box seat.
Hafeez then hit the winning runs in the final over to secure the win and the series.
"Credit to the way Pakistan played. They've got a very clear pattern which they executed for their last few matches. They did play very well," Williamson said.
"The first half was clinical from us and a good score on that surface and we weren't far away with the ball. As we've seen in the last game and this game it's really small margins. You could nitpick and look at two balls here or a couple of deliveries when we were batting but it's important to be a little bit better all round."
The third T20 match is on Monday.