By Scott Malone
Right wing Miroslav Satan tipped in the winning goal on a power play after the Sabres were found to have too many players on the ice.
The best-of-seven series will continue in Buffalo on Friday.
"That's a good feeling and you know the series is not done," said Satan after the frenetic contest in which players were pulled for engaging in a punch-up.
"The last game is always the toughest and you know they are going to be desperate at home. We're just going to see probably what we saw today, more of it. It'll be a very even game and the series will continue."
The Sabres stole the early running, scoring just two minutes into the first period through left wing Tim Kennedy, before Steve Montador made it 2-0 seven minutes into the second.
The scoreles Bruins came out of the locker room pumped up with aggression for the third, however, and were rewarded when Bruins center David Krejci turned the momentum with a goal two minutes in.
Patrice Bergeron then scored another four-and-a-half minutes later to level the score at 2-2.
"We played a team that played extremely well tonight, we played a team that played desperate, that played hard. And if we were going to win this game we needed to show a little more urgency. The guys came out and we did the job."
Sabres defenseman Craig Rivet and Bruins left wing Milan Lucic were pulled off the ice for five minutes each in the second period after a punching match.
"Where we let down a little bit was the outnumbered rushes we had in the second period when it was 2-0 and we didn't shoot the puck," Ruff said. "I think we could have put the game away there."
(Reporting by Scott Malone; Editing by Ian Ransom)