OTTAWA — After a stunning meltdown late in the third period that led to a demoralizing 6-5 loss to the Senators in double overtime Saturday, the Rangers return to New York down 2-0 in their Eastern Conference semifinal series.

Senators forward Jean-Gabriel Pageau delivered the telling blows at Canadian Tire Centre. He scored the last three goals of the game, including his fourth of the contest on an ill-advised play by Rangers defenseman Nick Holden that led to an odd-man rush and a goal at 2:54 of the second overtime.

Pageau capped a two-on-one by beating Henrik Lundqvist with a hard wrist shot from just inside the right faceoff circle. The winning goal gave the Senators their first lead of the game.

Leading 5-3, the Rangers allowed two goals in the final 3:19 of the third period. With Ottawa goalie Craig Anderson pulled for an extra skater, Pageau’s third goal of the game tied the score at 5-5 with 1:02 remaining.

Pageau, who scored 12 goals in 82 regular-season games, is only the second player in NHL history to score four goals, including the overtime winner, in a playoff game. The Ducks’ Joffrey Lupul did it in 2006 in a win over the Avalanche. Pageau also is the first player with four goals in a playoff game since the Red Wings’ Johan Franzen did it against the Sharks in 2010.

Said Dan Girardi: “Not much we can do right now. That one hurts a lot. Maybe it’s good we have a couple days between games. Maybe stay away from the rink and get our heads right and use the crowd to our advantage on Tuesday. This is a tough one for us to believe; we had a good effort for the most part. It’s hard to describe.”

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Rangers coach Alain Vigneault, who looked weary afterward, insisted that there wasn’t a time when he felt the game was slipping away.

“We didn’t do what we needed to do, and they made the most of their opportunities,” he said. “We spent a lot of time in their end. We had some good looks. They got a couple tips that found the way to the back of our net. We started out by giving them three power plays, made some adjustments and we were all right. We played a strong game. Just came up short.”

To be sure, Lundqvist (28 saves on 34 shots) was not at his best. He said afterward that he was not moving as well as he had in previous games. “I need to make that extra save at the end. That’s my job,” he said. “I know it’s tough to play deflections, but I’ve got to find a way.”

The Rangers killed those early power plays and took a 1-0 lead on Michael Grabner’s shorthanded goal at 4:16 of the first period.

Pageau scored his first goal when he scooped up a wayward cross-ice pass by Girardi and fired a wrister from a sharp angle outside the right faceoff circle that zipped over Lundqvist’s left shoulder at 13:59 of the first period to tie it at 1.

The Rangers went ahead 3-1 in the second on goals by Chris Kreider, who scored his first goal of this postseason, and Derek Stepan, who scored his first non-empty-net goal of these playoffs. Stepan’s goal also came with the Rangers shorthanded.

There were four goals — three by the Rangers — in a 5:12 span of the second period. The Blueshirts outshot the Senators 19-10 and could have had more if not for some remarkable saves by Anderson (43 saves), who also stopped 11 shots in overtime.

Brady Skjei’s first goal, a long, soft shot that went through traffic and Anderson, made it 4-2 at 15:51 of the second. Skjei’s second goal forged a 5-3 lead at 5:10 of the third that appeared as if it might be enough of a cushion. It wasn’t.

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Vigneault shortened his bench late, going with veteran forwards, not using Pavel Buchnevich and Oscar Lindberg in overtime and using Jimmy Vesey for just 2:13. Said Vigneault, “I felt at the time that we had a couple [other] guys who were playing pretty good.”