“I thought it was over,” said Dennis Seidenberg. He was far from alone. It was Game 7 of the first round, and the Boston Bruins faced a 4-1 deficit in the third period against the Toronto Maple Leafs. Fans were abandoning their seats at the Garden. On the bench, Milan Lucic was imagining the end of the group that won the Stanley Cup in 2011 – firings, trades, changes. In the dressing room, nursing an injury, watching on TV, Seidenberg’s mind drifted to the same dark place.

“I was worried about what was going to happen to this team – and everybody,” Seidenberg said. “It was all bad feelings and bad thoughts.”

Remember this as the Bruins and the Chicago Blackhawks battle in the Cup final. Both are recent champions, and both are rolling. But both have been through near-death experiences in these NHL playoffs, and both have been born again because of them. It’s not just that they survived; it’s how they thrived afterward. It’s why they are here. It’s why no one should change the channel now no matter the score, no matter the situation, no matter the history. Just when you think the end has come, a gutsy play, a fluky bounce or a little mistake could be a brand-new beginning.

It should have been over for the Bruins. They were reeling. They had blown a 3-1 series lead to the Leafs, and no one in NHL history had come back from a three-goal deficit in the third period of a Game 7. Only one team in NHL history had come back from a two-goal deficit in the third period of a Game 7 – the Montreal Canadiens, against the Bruins, in 1979.

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But then Lucic rumbled down the left wing, wrapped around the net and threw the puck out front. Nathan Horton fired a wrist shot from the left circle through a clear lane with 10:42 to go. It was 4-2. “I’m like, ‘Oh, there’s a chance,’ ” Seidenberg said.

“You could feel the momentum after we scored the second goal,” said Bruins general manager Peter Chiarelli. “I’ve never seen something like that happen, with two [extra-attacker] goals like that. But I could feel it coming, and I could see the Toronto team see that it was coming, too. Whether it was going to happen or not, as the minutes progressed, I thought, ‘This might happen.’ ”

The Bruins pulled their goalie in the final moments. Zdeno Chara fired a point shot. A rebound popped to Lucic in front. Lucic swiped in the puck and looked up at the clock as he celebrated. There was 1:22 to go. It was 4-3. “I was like, ‘Well, now it’s getting tight,’ ” Seidenberg said. Bergeron fired a point shot through traffic. There were 50.2 seconds left. Well, now it was tied.

The fans who stayed stood and sang before overtime – “Don’t Stop Believin’,” “Livin’ on a Prayer,” “Shipping up to Boston” – and they roared 6:05 into overtime. The Bruins pressured and they took a shot and a rebound squirted out and the Leafs couldn’t clear and the puck sat there for Bergeron, and then it was over. For the Leafs. “It was crazy,” Seidenberg said.

“After the game, I think it really opened our eyes to how hard we had to play and how hard we had to battle if we wanted to continue,” said forward Brad Marchand. “We knew we had something special in here, and we didn’t want to take it for granted.”

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The Bruins have lost once since. They took a 3-0 lead on the New York Rangers in the second round and closed them out in five. They swept the Pittsburgh Penguins in the Eastern Conference final, holding the top offensive team in the league to two goals – and holding Sidney Crosby, Evgeni Malkin, Kris Letang, Jarome Iginla and James Neal to zero points.

“You can tell in the way we’ve been playing since that game that we were able to create some momentum, and it carried on into the New York series, and it carried on to (the Pittsburgh) series,” Lucic said. “I think once we won that game, we definitely started to believe in what we could accomplish. And here we are.”

* * * * *

The Blackhawks didn’t blow a 3-1 series deficit. They didn’t face a 4-1 deficit in a Game 7. But they did face a 3-1 series deficit, and they did trail in the third period of an elimination game and did go to OT in a Game 7.

“They were on the ropes in one game,” said Blackhawks coach Joel Quenneville. “We were on the ropes for three.”

A 3-1 series deficit ain’t what it used to be. Teams seem more capable of and more confident in coming back in this era of the salary cap and parity. But when the Blackhawks were down to the Detroit Red Wings in the second round, no one knew what to expect. They had set an NHL record by earning points in their first 24 games – half the lockout-shortened schedule – and cruised to the Presidents’ Trophy as the top regular-season team. Though the 2010 ’Hawks had trailed in two series on the way to the Cup, this group was different, and this group had yet to be tested. Rare is the team that waltzes through the playoffs like the Los Angeles Kings did last year, taking four 3-0 series leads and finishing 16-4.

“We played great hockey throughout the regular season,” said captain Jonathan Toews. “We were able to get to the top of the league and stay there for most of the time. You know that you’re not going to go through a lot of things you go through throughout the playoffs. I don’t think you get a chance to play for the Stanley Cup without going through moments like we have. You got to persevere.”

The ’Hawks didn’t think it was over. After they lost Game 3 in Detroit, defenseman Duncan Keith slouched against the cinderblock wall in the dressing room. He spoke quietly. “I think it’s good for us,” he said. After they lost Game 4 in Detroit, too, winger Patrick Kane said people were counting them out. Even if the vast majority weren’t, that was the attitude.

After a dominant victory in Game 5, the Blackhawks found themselves facing a 2-1 deficit in the third period of Game 6. They were 20 minutes from elimination. Two defensive mistakes by Detroit and a penalty-shot goal saved them. Late in regulation of Game 7, they scored what looked like the winning goal, only to have it waved off because of a penalty call and go to overtime. They were one goal – one shot, one goof, one bounce – from elimination. But they got the bounce when Brent Seabrook fired a shot off a shinpad and into the net 3:35 into OT.

“I think it’s a very fine line,” Toews said.

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The Blackhawks have lost once since. They beat the Kings in five games in the Western Conference final. They were just about to close out Game 5 when Mike Richards deflected a shot into the net with 9.4 seconds left in regulation, forcing overtime. Uh-oh. “So emotional,” Kane said. But the ’Hawks talked about how a break had gone against them late in regulation in Game 7 against Detroit and they had won, anyway, and Kane scored 11:40 into double OT.

“Coming back from that 3-1 deficit against Detroit, I think that gives you a lot of confidence,” Toews said. “Here we are again. We got a great chance to go back to where we want to be. We know there’s going to be some more tough moments that we’ll have to battle through. We’re confident we can do that as a team.”

* * * * *

The Bruins and Blackhawks are both Original Six teams. Both have won the Cup in the past four years – the Blackhawks in 2010, the Bruins in 2011.

Both have veteran, accomplished coaches who have been fired in the media since but not in reality. Both have the same core players but new goalies putting up better numbers than their predecessors (Tuukka Rask replacing Tim Thomas in Boston, Corey Crawford replacing Antti Niemi in Chicago). Both have Norris-winning defensemen (Keith for Chicago, Chara for Boston). Both have Selke-caliber defensive forwards (Bergeron for Boston, Toews for Chicago). Both have deep lineups. Both have puzzling power plays and strong penalty kills.

The Blackhawks are known more for their skill and offense; the Bruins are known more for their grit and defense. But the Blackhawks just beat an L.A. team known for its grit and defense, and the Bruins just beat a Pittsburgh team known for its skill and offense. The Blackhawks are better defensively and the Bruins better offensively than they are given credit for. Both dominated two series outside of a seven-game scare. The Blackhawks are 12-5 and have won seven of their last eight; the Bruins are 12-4 and have won nine of their last 10.

This series could come down to how well the Blackhawks’ defense moves the puck, diffusing the Bruins’ forecheck, or how well the Bruins pin the Blackhawks in their end, keeping them from getting started. It could come down to the Blackhawks’ excellent transition game or how well the Bruins play in the neutral zone. It could come down to how Toews and Kane and Marian Hossa and Patrick Sharp produce, or if the Bruins shut them down the way they shut down Crosby and Malkin and Letang and Iginla and Neal. It could come down to an unappreciated player like Boston’s David Krejci, the playoffs’ leading scorer once again, or a surprising player like Chicago’s Bryan Bickell, who has eight goals. It could come down to Rask or Crawford or something we haven’t considered.

But it will come down to holding a lead or coming back from a deficit. It will come down to survival. And it will not be over until Toews or Chara lifts the Cup over his head for a second time.

“We know in the final we’re going to face another former Stanley Cup champion team,” Krejci said. “We know it’s going to be tough. But who cares?”

They have faced elimination, and they have survived. They are facing their equals, and they have never felt more alive.

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