They might even wind up back in the same arena next week.
Sergei Gonchar scored a power-play goal 47 seconds into overtime on Karlsson's second assist of his successful and sooner-than-anticipated return from a torn Achilles tendon, and Ottawa clinched a playoff berth by beating the Washington Capitals 2-1 on Thursday night.
"I would have been happy if I sat in the stands and we would have won this game, as well, I think, and that's the main goal," Karlsson said. "I'm going to have to work on my stuff, and as long as this team keeps winning, I'm going to be happy."
"I'm just glad to put all the doubters behind us," said Craig Anderson, who made 19 saves. "We lost a couple games there ... and I think that was kind of the talk: Are we going to fold the tent and go home? But I think we all kind of strapped on the belt and went to work and found ways to make people believers."
The only shot that got past him was Alex Ovechkin's NHL-leading 32nd goal, which tied the game 8½ minutes into the third period.
But ex-Capitals defenseman Gonchar took a pass from Karlsson and put the winner past Michal Neuvirth, who made 39 saves.
The Southeast Division champion Capitals had nothing at stake. They already are locked into the No. 3 seed in the Eastern Conference.
The Senators, who have two games left, moved up to sixth in the East. If they remain there, Ottawa would face the Capitals, who have won 10 of 12 overall -- with the only losses against the Senators.
"If we end up playing them, we'll be ready," Washington's first-year coach Adam Oates said.
Ottawa won all three regular-season matchups against the Capitals.
"We just came out flat," Washington defenseman Mike Green said. "I don't know what it is the last couple times we've played against them. But it's the way it's gone."
The Capitals secured their sixth consecutive playoff appearance by clinching the division title with a 5-3 victory over Winnipeg on Tuesday.
So while the Capitals didn't need a victory on Thursday, the Senators most certainly did.
"It was a big game," Neuvirth pointed out, "for them."
For Karlsson, last season's Norris Trophy winner as the NHL's top defenseman, this was his first game since Pittsburgh's Matt Cooke stepped on the back of his leg on Feb. 13.
Karlsson originally was expected to miss the rest of the season after surgery, but recently returned to practice. He sat out 31 games, but now is back in time to get into game shape ahead of the playoffs.
"I felt OK," Karlsson said. "Did battle some issues out there, and didn't feel quite as comfortable as I'm used to, but overall I think it was a solid game. I still have to work through some mistakes and clean those up."
Karlsson played a game-high 27-plus minutes and managed to demonstrate just how valuable he is.
With the game scoreless 12:35 into the second period, from barely inside the blue line, he shook his stick to get the attention of Milan Michalek, whose pass was strongly one-timed by Karlsson toward the net. The puck was redirected by Jakob Silfverberg. Then came the set up for Gonchar.
"Obviously, our team was a different team with him on the ice, and the things that he can do that others can't do. He's a very special player," Senators coach Paul MacLean said about Karlsson. "He's done, obviously, a ton of work."
Silfverberg briefly stayed down on the ice with about three minutes left after appearing to take a glove to the face from Capitals defenseman Steve Oleksy. No penalty was called.
But Capitals forward Mike Ribeiro was sent to the box twice in the final five minutes of regulation, and teammate Jason Chimera was given a 10-minute misconduct in the last minute. The game was chippy throughout, including a scrum involving eight players along the boards earlier in the third period. The first period had a handful of near-fights and some late hits.
Ovechkin scored his fourth goal in three games, but his first point against Ottawa this season.
He gathered the puck near the boards on the right side -- Oates moved him from left wing -- and swept in front of the net with defenseman Chris Phillips riding his back, before flipping a backhander past Anderson.
Nearly seated on the ice afterward, Ovechkin spun around and kissed his glove, while fans responded with loud chants of "M-V-P!"
It was Ovechkin's 600th NHL game. ... Ottawa will play at home against Philadelphia on Saturday, and at the Bruins on Sunday in a game rescheduled because of the Boston Marathon bombings. ... The Capitals finish the regular season Saturday against visiting Boston.
Copyright by STATS LLC and The Associated Press
WASHINGTON, DC - APRIL 25: Erik Karlsson #65 of the Ottawa Senators warms up before an NHL game against the...
(Photo by Patrick McDermott/NHLI via Getty Images)