Atlanta closed within three points of Boston for the final playoff spot in the Eastern Conference after the Bruins lost to Pittsburgh 3-0. The Thrashers wasted a pair of two-goal leads for the second game in a row, but still managed another vital win.
"We feel a lot better about ourselves than we did three or four days ago," Atlanta coach John Anderson said.
Antropov scored off a pair of deflections in front of the net, including a tiebreaking power-play goal with 16:04 remaining in the third period.
Armstrong added two goals less than 5 minutes apart to put it away for the Thrashers, who had lost six in a row before beating a pair of teams that appear headed for the playoffs, Buffalo and Ottawa.
"These are not bad teams were are beating right now," Anderson said. "It gives you a little bit of a pick-me-up."
The Senators could use a boost. They have lost seven of eight games since the Olympic break, including the last four.
"We're making mistakes," said Chris Phillips, who scored for the Senators but also had a crucial turnover. "We're making it easy for the other teams to score goals."
Atlanta broke a 3-3 tie after Andy Sutton, who had a rough night, delivered a cheap shot to Eric Boulton near center ice and was sent off for elbowing. With a man advantage, Tobias Enstrom fired a slap shot that Antropov deflected past Brian Elliott for his 21st goal of the season.
Armstrong clinched it for the Thrashers. First, he stole an attempted clearing pass by Phillips and ripped a shot over Elliott's left shoulder for an unassisted goal with 14:26 left. Then, after Armstrong had a shot blocked in front, he scrambled toward the net, took a return pass from Jim Slater and flipped it past Elliott with 9:40 left.
Atlanta scored on its first three shots on goal in the third period.
"We're chipping away at it," Armstrong said. "Hopefully it will all work out for the best."
The Thrashers had leads of 2-0 and 3-1 before the Senators tied it with two goals just 43 seconds apart late in the second period.
Atlanta goalie Johan Hedberg was livid about the first one, believing he was interfered with by Jarkko Ruutu after stopping the initial shot by Chris Campoli. The puck slid wide of the post, and Ruutu knocked over Hedberg at the top of the crease going after the loose puck.
Hedberg couldn't get back in time as Ruutu scooped up the puck from behind the goal line, then tucked it inside the post for his 11th goal. The goalie complained bitterly and had to restrained by a linesman as he attempted to get at referee Stephane Auger.
"He couldn't get back to the net and he was elbowed in the face," Anderson said sarcastically. "Other than that, the ref said it was a good hockey goal."
Still seething about that call, Hedberg got even angrier when Phillips picked off an errant clearing pass and tied it up with a screened shot.
But Hedberg came up big most of the night. Among the 36 shots he stopped was a brilliant pad save on Jason Spezza, who cut in alone midway through the third looking for a goal that could have made it a one-goal game. Instead, Atlanta took off the other way for Armstrong's goal that stretched to a three-goal lead.
Just 2 1/2 minutes into the game, Sutton let a long clearing pass slip between his legs as he backed up near the Ottawa blue line, allowing Evgeny Artyukhin to score off a breakaway. Slater scored for the second game in a row to make it 2-0 early in the second.
After Chris Kelly's goal halved the lead, the Thrashers stretched it back out to a two-goal advantage on Antropov's first of the night. He got his stick on a shot by Pavel Kubina that likely would have gone in anyway, since Elliott was scrambling to get back in position after making a difficult save at the side of the net.
Atlanta killed off four penalties, snapping a streak of 11 straight games giving up at least one power-play goal. ... The Thrashers won the season series 3-1, scoring six goals in each of the last two meetings. Atlanta beat the Senators 6-1 on Jan. 12. ... Ottawa outshot the Thrashers 39-27.
Copyright by STATS LLC and The Associated Press
Arena: Philips Arena Location: Atlanta, Georgia
Referees: Stephane Auger, Brad Watson Linesmen: Michel Cormier, Steve Miller
Attendance: 12,718 (68.6% full)
Team Stat Comparison
Power Play Goals
Power Play Opportunities
Shots on Goal
Evgeny Artyukhin (6)
Assists: Johnny Oduya, Ron Hainsey
Jim Slater (10)
Assists: Todd White, Colby Armstrong
Chris Kelly (13)
Assist: Milan Michalek
Nik Antropov (20)
(Power Play) Assists: Pavel Kubina, Maxim Afinogenov
Jarkko Ruutu (11)
Assist: Chris Campoli
Chris Phillips (8)
Assists: Jason Spezza, Daniel Alfredsson