"The game's fast and he's our fastest player," Senators coach D.J. Smith of Duclair. "He's making plays, certainly he's confident right now, and he's been reliable. Sometimes that's with confidence, sometimes with age, but he's been good for us all year."
Chris Tierney had a goal and two assists, and Anders Nilsson made 38 saves for the Senators (13-17-1), who had lost six of their previous seven games.
[WATCH: All Bruins vs. Senators highlights]
It was Ottawa's first win against Boston since a 2-1 shootout victory on April 6, 2017 (0-9-1 in previous 10 games).
"For sure they are (the best team in the League)," Smith said. "They're ready to win now and they came within a game of winning the Stanley Cup (last season). They have some high-end players and some guys that know how to win over there."
Patrice Bergeron scored in his return, and Brad Marchand and David Pastrnak each had two assists for the Bruins (20-5-6), who are 0-2-1 in their past three games after an eight-game winning streak. Tuukka Rask made 23 saves.
"We're going through a tough stretch in terms of getting our energy level where it needs to be," Boston coach Bruce Cassidy said. "If you're not competing hard on the puck, around the puck, on the puck, in this League, you're not going to beat anybody. Ottawa was just better than us. They won more races, they were more competitive early on. Now we eventually got to our game, but again, it's too late. We knew this would catch up to us, so another kind of learning experience for us."
Artem Anisimov gave the Senators a 1-0 lead at 1:35 of the first period, receiving a stretch pass from Dylan DeMelo and skating in on a breakaway before scoring with a backhand.
Video: BOS@OTT: Anisimov roofs backhander on breakaway
Duclair made it 2-0 at 15:44 with a wrist shot from the high slot. He has seven points (five goals, two assists) in a four-game point streak.
"Everyone's been buying in for a while now, and it's nice to see," Duclair said. "You guys see it from up top, we're sacrificing our bodies, and that gives us momentum. It's nice to see."
Bergeron scored into an open net with a one-timer off a pass from Pastrnak on a 2-on-1 to cut it to 2-1 at 17:48.
"I felt good," said Bergeron, who had missed the previous seven games because of a lower-body injury. "I was trying to keep things simply early on and then I got going, felt better, got my legs underneath me and felt pretty good."
Video: BOS@OTT: Bergeron places one-timer into open net
After Rask misplayed the puck behind the net, Vladislav Namestnikov collected the turnover and passed to Tierney, who scored into an open net before Rask could scramble back into position to make it 3-1 at 1:21 of the second period.
"I thought it was our [defenseman], so I kind of just let it go, thinking that he was going to rim it," Rask said. "Clearly, it was the wrong play on my part. It's one of those that I've got to be more aware of and communicate with my [defensemen]."
Jean-Gabriel Pageau scored a shorthanded goal into an empty net to extend the lead to 4-1 at 17:02 of the third period.
Jake DeBrusk jammed in a loose puck during a scramble in the crease to cut it to 4-2 at 17:47, but Duclair scored into an empty net at with one second remaining for the 5-2 final.
"It's been happening for a while now, that we fall behind in games. I think we want to correct that," Rask said. "A lot of it is on me. When I play, I want to give us a chance to get the lead and not fall behind one or two goals, in the first period, especially. That never helps. Then again, we battled back and we kind of found our game late in the game. But sometimes, you play with fire when you fall down a few goals and you're trying to catch up."
Video: BOS@OTT: Tierney capitalizes on Rask's error
They said it
"At the start of the year, our power play was carrying us. On a night like tonight, we'd probably get two on it earlier in the year and it ends up being the difference in the game, and maybe a timely save mixed in there. Now you put those two things not going our way, and all of a sudden we're in the loss column." -- Bruins coach Bruce Cassidy
"It's great, as a goalie, to stand back there and see when the guys are playing so hard and sacrificing themselves and blocking shots and battling through it even though they're really tired." -- Senators goalie Anders Nilsson
Need to know
The Bruins went 1-for-5 on the power play. ... Ottawa had 29 blocks compared to six for Boston.
Bruins: At the Washington Capitals on Wednesday (7 p.m. ET; NBCSN, TVAS)
Senators: At the Montreal Canadiens on Wednesday (7:30 p.m. ET; SN, RDS, NHL.TV)
Video: Duclair posts three points, Nilsson stops 38 in win