Florida outshot Ottawa 28-15 at even strength but finished 0-for-4 on the power play.
"For the most part, we put together not a bad performance," Matthew Tkachuk said. "I thought we were pretty good most of the night. It was just, we got into some penalty trouble and they capitalized on their power plays, and we didn't on ours. They definitely crushed us in the whole special teams battle tonight."
Brady Tkachuk gave the Senators a 1-0 lead at 9:02 of the first period. Thomas Chabot deked Panthers forward Givani Smith at the point and sent a cross-ice pass for a one-timer.
"I thought that's as mature a game as Brady's played in a long time," Ottawa coach D.J. Smith said. "I think he was winning inches there at the end, and that's what you have to do. This time of year, these are playoff games, essentially us versus them, and every inch matters."
Brassard made it 2-0 at 18:55 when he redirected Erik Brannstrom's one-timer of a loose puck in the slot on the power play.
"They have skilled guys on that power play," Florida forward Eetu Luostarinen said. "We took a couple of stupid penalties, including myself. We just have to stay out of the box."
Forsling's slap shot from the point beat a screened Sogaard to cut it to 2-1 at 5:58 of the second period.
"He's been so good all year," Panthers coach Paul Maurice said of Forsling. "Played in every game, and confidence is growing so strongly. He only just keeps getting better. He's got a great shot. We're just starting to see it, and I think you're going to see a lot more of it."
The Senators extended their lead to 3-1 at 19:22. DeBrincat finished a tic-tac-toe passing play, one-timing a no-look feed from Drake Batherson on a power play.