But Brownlie fell to Shahid Afridi and Devcich was run out before Ross Taylor, Tom Latham and Corey Anderson fell in quick succession to leave New Zealand struggling on 167 for five.
Williamson and Luke Ronchi (36) shared a sixth-wicket partnership of 58 to put the tourists back on course and the experienced Daniel Vettori (13 not out) helped steer his side to their target with four overs to spare.
Opener Mohammad Hafeez top-scored for Pakistan with 76, including a six and nine fours, as the hosts recovered from a poor start in which they slumped to 20 for three.
Pakistan captain Misbah-ul-Haq then chipped in with 47 runs and Haris Sohail made 33 to lift their side to a respectable total of 252.
New Zealand seamer Matt Henry returned figures of four for 45 in his third one-day international.
The third game in the five-match series is in Sharjah on Sunday.
(Reporting by Ed Osmond; Editing by Ken Ferris)
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