Southee claimed five wickets for 8 balls, finishing with 5/17 -- the first five-wicket bag by a New Zealander in Twenty20 Internationals -- as New Zealand held Pakistan to 143/9 after winning the toss.
Guptill then made 54, including a 23-ball half century -- the fastest by a Kiwi in T20s -- to guide New Zealand to victory with 17 balls to spare.
Southee set up the win, which puts New Zealand 1-0 up in the three-match series, with an outstanding spell of swing bowling.
