New Zealand took four leg byes from the fourth ball of the 18th over to win the match with eight balls to spare. In doing so Guptill and Williamson surpassed the Twnety20 record partnership for all wickets against all nations that had been held for South Africa by Graeme Smith and Loots Bosman at 170.
Pakistan won the opening match by 16 runs on Friday, defending a total of 171-8 and left itself in a similar position at Seddon Park when it made 168-7 after winning the toss and batting.
Williamson ran out Guptill for 2 in one of the most serious blunders of the first match at Eden Park and there seemed a clear determination from both batsmen to avoid similar missteps this time around.
Guptill and Williamson controlled New Zealand's chase, posting their century partnership in 12 overs to ensure the home team remained ahead of the required run rate and to leave Pakistan with few defensive options.
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There were moments during the New Zealand innings when Pakistan seemed set to slow New Zealand's scoring and when Guptill showed signs of concern. But Williamson helped him and New Zealand through those moments and eventually Guptill became the more vigorous of the two, reaching his half century from 40 balls and finishing with 87 from 58 balls with nine fours and four sixes.
The boundaries at Seddon Park are relatively short, particularly square on the off side and Williamson punched anything short to boundaries that could not be adequately guarded. Williamson reached his half century from 35 balls and finished with 72 from 48 balls with 11 fours.
"I think we made those improvements that we wanted to from the last game," Williamson said. I thought the bowling effort was pretty good considering that short side and the wind pushing the ball to the long side, so it wasn't easy.
total when it came in after batting on winning the toss with 168-7, just short of its winning tally in the opening match.
Umar Akmal provided the backbone of the innings with a superb 56 from 27 balls that included four fours and four sixes. He had support from Shoaib Malik who made 39, Mohammad Hafeez with 19 and Shoaib Maqsood with 18.
New Zealand took an innovative approach to containing the Pakistan batsmen, opening the bowling with left-arm spinner Mitchell Santner and allrounder Corey Anderson. While the tactic didn't work emphatically, it helped curb Pakistan's total.
"That was a small plan. I guess it didn't come off today but that's Twenty20 cricket."
The teams next head to Wellington for the deciding match of the series on Thursday.
"I think I don't have any words to describe the match but I think they played outstanding cricket," Pakistan captain Shahid Afridi said.
"Especially Guptill and Williamson, they played very well, they played extraordinary.
"I was happy, the team was happy with our total but the one thing, we didn't bowl with enough variation and that was the thing we missed.