The Flyers had a chance to eliminate Atlanta from a playoff spot.
The Eastern Conference's top team instead left the door open a crack for the pesky Thrashers to slither into the postseason and get another chance at punishing Philadelphia.
Nik Antropov scored the only goal, and Mason stopped 43 shots for his first shutout of the season in the Thrashers' 1-0 win over the Flyers on Thursday night.
Mason earned his 21st career shutout and was sensational against the Eastern Conference-leading Flyers. He stopped 14 shots in the second period and got some timely offensive help for his 13th win of the season.
Antropov scored his 15th goal midway through the third period and it stuck to keep Atlanta's faint playoff hopes alive.
''The trouble with us is, we seem to have not been ready for teams that are near us in the point standings,'' Mason said. ''We come into a place like this knowing we have to be ready or we'll get our butts kicked. I don't know what it is. Maybe we just get more prepared.''
The Thrashers crashed after a 20-15-6 start and will need to win their final five games and get some help to make the postseason for only the second time in team history. They would have been eliminated with a loss.
Atlanta went 3-1 vs. Philadelphia this season - good enough to take a commanding lead in a potential best-of-seven series. The Flyers have lost 10 of 16 and lead the Washington Capitals by one point - 102-101 - for first place in the conference. The Flyers went 0 for 5 on the power play.
The Flyers continue to struggle without defenseman Chris Pronger in the lineup. He had surgery earlier this month on his broken right hand and their home-ice swoon started around the same time.
Flyers general manager Paul Holmgren said Pronger had a ''minor setback'' and is out until the playoffs.
''We're not going to be real aggressive at this point,'' Holmgren said. ''Obviously, the playoffs are more important right now.''
Holmgren said Pronger has done little rehab on his hand the last few days. Holmgren said doctors expected Pronger at 100 percent for the playoffs. The Flyers finish the regular season April 9.
Mason was outstanding in the final minute. He made one kick save off his left leg and Jeff Carter's quick rebound knocked off the post with 4.7 seconds left to help Atlanta hang on.
''Now's the time of the year where you're going to score those goals in front of the net on second and third opportunities,'' Flyers captain Mike Richards said. ''We just have to be a little bit more hungry for those opportunities.''
The Flyers, who pride themselves on their intimidating home-ice advantage, have lost five straight at home. The Thrashers started the streak March 12, then came consecutive shootout losses, and a 2-1 loss to Boston on Sunday night.
''It's probably more disappointing and frustrating than concerned,'' Flyers coach Peter Laviolette said.
The Flyers might wish they could pass on the home-ice edge advantage that would come in the postseason with a No. 1 seed. They are a sparkling 25-9-4 on the road.
The Thrashers would have to feel good about pulling off a postseason upset if they could sneak into the Eastern Conference playoffs as the No. 8 seed.
Atlanta snapped the scoreless game with 9:38 left in the third. The Thrashers didn't solve Sergei Bobrovsky as much as they caught a break - Evander Kane's pass cross-crease pass to Antropov shot off the skate and into the net.
The goal stood after a brief review showed Antropov did not make a kicking motion.
''I drove to the net, I was fortunate it hit me,'' Antropov said. ''I was just trying to stop, but it hit my skate.''
For all their success, the Flyers have yet to get a shutout this season. Brian Boucher had the last one on April 6, 2010, vs. Toronto.
Mason got his - and he'd love one more shot at them in the playoffs.
''We came in knowing how good these guys are and that they are going to come out hard in this building,'' he said. ''I made some big saves early and that makes the guys feel good.''
Notes: The Flyers honored former captain Bill Barber in a pregame ceremony. He scored 420 goals and had 883 career points and helped lead them to two Stanley Cup championships. ... Atlanta is 7-4-1 since March 5 to make its late playoff push.
