"First goal is tough when it comes right off the bat, and he opened me up. It's something I've got to work on," Lehner said. "I think the tip on the third goal, too. It's just a reflex to start reaching with your stick to deflect it into the corner and out of the rink, and he tipped it back.
"That stuff bugs me and I've got to work on it, but I felt like I came back good in the third and had some good saves."
The teams meet again Tuesday in St. Louis for the second and final time this season.
Ottawa (9-6-4) was only 3-7-2 in afternoon games last season, and the Senators repeated the trend against the Blues.
Karlsson laughed off that notion.
"It's just another stat to keep track of," he said. "We haven't won a lot of afternoon games, we're all aware of it, but it's not something we really think about. We can try to do different things, but we try everything we can to win games.
"They're a good team, and we're going to lose sometimes."
Trailing 3-2, the Senators started the third period on a power play, but were unable to capitalize. They were outshot 15-5 in the frame.
"They outplayed us in the third period, which was the only thing that would be disappointing in the game," Senators coach Paul MacLean said. "I thought we were competitive and played hard against a hard team."
The teams exchanged goals in the second, but Lehner and Elliott stood out.
Elliott made a glove save against Colin Greening early in the period, and minutes later Lehner bailed out Karlsson following a giveaway that led to a point-blank chance for Tarasenko.
It was the second time in the game that Lehner covered after a giveaway by the captain.
The Senators cut the deficit to one on Karlsson's power-play goal from the point with 4:39 left in the period.
"I didn't see it coming through all the bodies," said Elliott, who spent parts of four seasons with the Senators. "Those are tough ones to swallow, especially when you're up 3-1. That goal makes it a tight game.
"We battled back and kept things to the outside for most of the rest of the game. When you're not going to the box, that helps, as well."
Both teams had a solid first period when the Blues took a 2-1 lead.
St. Louis opened the scoring on its first shot when Maxim Lapierre got around Cody Ceci and went on to beat Lehner through the legs at 4:16.
Ottawa tied it with a power-play goal at 16:23 as Bobby Ryan came from behind the net and found Zibanejad in front for a shot that quickly beat Elliott.
Just 30 seconds later, St. Louis regained the lead as Jori Lehtera found Tarasenko alone for his team-leading 12th of the season.
NOTES: Senators D was scratched after taking part in warmups. He was replaced by Patrick Wiercioch. ... D Marc Methot remained sidelined (back, indefinite). Erik Condra and Curtis Lazar were healthy scratches for Ottawa. ... The Blues' Joakim Lindstrom, Chris Porter and Chris Butler were healthy scratches.