Troy Brouwer scored on a power play with 1:27 left, giving the Capitals a 4-3 victory Saturday and a chance to eliminate the defending champions in Game 6 in Washington on Sunday.
The Bruins rallied from deficits of 2-0 and 3-2 before Brouwer beat Tim Thomas over the glove with a wrist shot from the right circle. Last year, Boston overcame 2-0 and 3-2 deficits -- in games -- in the Stanley Cup finals against the Vancouver Canucks.
"Everyone knows the fourth win is the hardest," Capitals goalie Braden Holtby said, "but we're not really focusing on whether the Bruins can come back. ... We have a game to win and we have a certain way we want to do it and that's to stick to our systems."
Thomas, last year's postseason MVP and Vezina Trophy winner, knows the Bruins can't rely on past comebacks to start another one.
"It's good to know that we've been in tough spots before and responded well," he said. "Having said that, we've got to do that."
Capitals defenseman Karl Alzner figures the best way to stop that is to score early.
"We've got to come out hard, try to put a little bit of doubt in their minds," he said.
Johnny Boychuk tied it at 3 with 11:13 left on a power play, putting a 50-footer from the left into the far side of the net.
He's glad the Bruins won't have time to dwell on the loss.
"You don't have any time to think about what you could have done differently for four days or three days," Boychuk said. "You just have to worry about what you can do tomorrow. That's what I think our whole focus is right now."
"It was a tough call at 2:50 (left) in the game," Pouliot said. "It's a grind out there and sometimes you get the call, sometimes you don't."
Brouwer said he saw a Bruins defenseman turn toward the middle of the ice, giving him a lane in which to shoot for his second goal of the series.
"We were getting buzzed a little bit and I think they had a little bit of momentum off their power play goal," Brouwer said. "So, for us to get a power play late, create a little bit of offense and ultimately get a goal, it's good."
Washington took a 2-0 lead in the second period on goals by Alexander Semin at 11:16 and Jay Beagle at 14:27. That seemed to be a sizeable advantage in a series that featured tight defense with each team scoring only seven goals in the first four games.
About 15 seconds before Semin's goal, Bruins defenseman Joe Corvo blocked a shot and fell to the ice. He struggled to his feet and didn't regain his normal stride before Semin scored. Bruins coach Claude Julien said play should have been stopped. He also was upset when Zdeno Chara was dazed after taking an elbow to the head without a penalty being called.
"On the winning goal, it's a very weak (penalty) call in my mind," he said, "and it ended up costing us the winning goal."
The Bruins rallied with two goals in 28 seconds late in the period to tie at 2. Dennis Seidenberg scored with 2:39 left on a one-timer from the right circle that got past Holtby. And Brad Marchand tied it with 2:11 to go when he poked the rebound through Holtby's legs after a shot from the right point by Boychuk.
Washington regained the lead 3:21 into the third period when Mike Knuble pounced on a rebound. Joel Ward shot from 50 feet on the right side and Thomas saved it. But the goalie steered the puck to the left side and couldn't slide over in time to stop Knuble's shot.
"We could have been down a little bit after the second period when we had a strong 2-0 lead and they came back with two quick ones that can put us on our heels and really boost them," Knuble said.
But now he and his teammates have the momentum -- and a chance to advance to the second round for the third time in four years. The last time they got to the third round was in 1998, when they beat the Bruins in the first round in six games and went to the finals where they lost 4-0 to the Detroit Red Wings.
"We'll be happy on the plane ride home. Guys will be in great spirits," Knuble said, "but when we touch down, when we get back to Washington, the prep will be, to a man, to every guy, going on right away. ... We're excited to go back to Washington to try and close this out."
Holtby finished with 34 saves, and Thomas stopped 28 shots.
Seidenberg's goal was the Bruins first in 84 minutes, 9 seconds since Rich Peverley scored in the first period of the Capitals' 2-1 victory Thursday night. ... Boston coach Claude Julien shook up his lines to try for more offense and it finally paid off late in the second period. ... Beagle's goal was his first career playoff point. ... Washington's 2-0 lead was the first two-goal advantage for either team in the series.