DiPietro bounced back from a pair of concussions to stop 32 shots in his first game in three weeks, and Marc-Andre Bergeron scored the go-ahead goal eight minutes into the third period to lead the Islanders to a 3-2 win over the top-seeded Buffalo Sabres on Saturday night.
Trent Hunter and Bruno Gervais also scored, helping New York tie the best-of-seven, first-round series 1-1, with Game 3 on Long Island on Monday.
The Sabres started sloppy, allowing New York to take a 2-0 lead 11 minutes in, and at times looked nothing like the efficient team that produced a 4-1 win on Thursday and the NHL's top record this season.
Defencemen Toni Lydman and Dmitri Kalinin scored for the Sabres, who finished the regular season with a league-leading 53 wins and 308 goals.
The Islanders seemed to be hanging on before Bergeron scored a power-play goal with 11:23 left - and six minutes after Kalinin tied the game during a scramble in front.
Bergeron's goal came on the Islanders' first shot of the period, when his slap shot from the right point beat Ryan Miller. New York forward Ryan Smyth contributed by providing traffic in front to distract the Sabres goalie.