Ottawa Senators head coach Guy Boucher isn't sure what constitutes goaltender interference anymore.

The Senators recovered from a 3-0 deficit and felt they could tie it up again when William Carrier made it 4-3 for the Vegas Golden Knights near the midway mark of the third period. But after Pierre-Edouard Bellemare's goal with eight minutes left to play withstood a challenge Boucher was at a loss for words.

"I don't know anymore," said Boucher. "All of us together, we just don't know anymore. I'll let them decide and that's it. I can't make it, whether it's our game or somebody else's game. I guess that's why I'm not paid for those decisions."

The Knights made it 5-3, the game's final, score, as Bellemare had a wide-open net after Ottawa goalie Craig Anderson got tangled with Carrier. The Senators challenged for goaltender interference, but the call on the ice stood, bringing the crowd of 15,213 at Canadian Tire Centre to boo in unison.

"It's human nature," admitted Boucher. "I was 100 per cent sure it was going to be goaltender interference, so it's just mind boggling so by the time you get rid of the frustration and all that it's two goals. If it's one goal it might have been different."

Jonathan Marchessault, Shea Theodore and Nick Holden helped the Golden Knights (7-8-1) build a 3-0 lead before Ottawa mounted its comeback. Marc-Andre Fleury stopped 27 shots.

"We played a real good game," said Vegas head coach Gerard Gallant. "The puck finally went in the net for us so that was a real key for us to get five goals. We're happy with that."

Matt Duchene, Ryan Dzingel and Thomas Chabot each had a goal as the Senators (6-7-3) tied it 3-3, while Craig Anderson had a busy night making 37 saves.

Trailing 3-1 to start the third, Ottawa tied the game by scoring twice in a span of 64 seconds. Dzingel made it 3-2 and right after Chabot had a wide-open net as Fleury was caught sprawling.

Vegas regained the lead midway through the period as Carrier won a battle along the boards, stepped around Mark Borowiecki and beat Anderson stick side.

"I thought we did some really good things," said Duchene. "Obviously when you tie it 3-3 in the third, it can go either way. We had the momentum so tough way to lose."

Thursday's game featured the return of Senators rookie Brady Tkachuk, who had missed the last nine games with a torn ligament in his left leg.

Vegas dominated much of the second and took a 3-0 lead late in the period as Holden jumped into the play and lifted the puck over Anderson's right shoulder. It was his first with Vegas.

The Senators gave Borowiecki credit for providing the energy for the comeback. Late in the second Borowiecki accepted a challenge from Ryan Reaves to drop the gloves and less than 30 seconds later the Senators got its first goal.

"When you look at a guy like Boro, he's a great leader for us and he just came back and he gets right in a fight with Reaves," said Chabot. "I think it shows you how much he cares about that team and we all appreciate it and it gave us a lot of momentum."

Ottawa finally solved Fleury at the 17-minute mark. Fleury poke checked Chabot, but the puck went right to Duchene.

Reaves has provided similar incentive for the Knights and is willing to play whatever role necessary to help his team.

"There's been a couple high hits on our team the last couple weeks," said Reaves. "Everybody knows that if you're going to do that you're going to have to answer the bell. Good on him for answering the bell, not a lot of guys will do that, but good on him. There's got to be a message sent because there's been a couple high hits on our team."

The Knights jumped out to a 2-0 first-period lead after scoring twice on the power play.

Marchessault opened the scoring at the 15-minute mark and with 20 seconds remaining in the period Theodore, from just inside the blueline, fired a puck through traffic. With the win Vegas swept the season series 2-0.

Notes: Ottawa LW Max McCormick was a healthy scratch. Vegas was without LW Erik Haula as he was injured Tuesday against Toronto.