But it was Cumiskey who moved the Avalanche into first place in the Northwest Division.
"I just took what they gave me," Cumiskey said. "I wasn't expecting it. I don't believe I did it."
In overtime, Stastny won the faceoff and got the puck to Cumiskey, who skated into the Thrashers' zone. He split defensemen Zach Bogosian and Tobias Enstrom in front of the net and beat Johan Hedberg for his seventh goal of the season.
"He got the puck from the faceoff and pretty much skated through our whole team and scored," Hedberg said. "I don't think anyone expected him to keep going like he did. It was a great play by him."
The play didn't sit well with Atlanta coach John Anderson.
"I saw two defenseman not taking anybody in front of the net," he said. "They let him walk right in. I've never seen that in my life before."
Anderson's real disappointment was that the Avalanche had a chance to win it in overtime. The Thrashers were carrying the play, and goals by Nik Antropov, Niclas Bergfors and Bryan Little gave them a 3-1 lead with 15:41 left in regulation.
But Colorado battled back to force overtime. Stewart knocked in his own rebound with 11:26 left, and Durno beat Hedberg with 5:29 remaining in regulation.
Hedberg, who had 31 saves, thought Durno's goal shouldn't have counted because Cody McLeod kept the puck alive with a high stick.
"I couldn't believe they didn't blow that down," he said. "If it doesn't go straight in, it's reviewable. It's up to the ref to make a stick decision on that."
Colorado coach Joe Sacco said Durno's goal was the turning point.
"Cumiskey's goal was a great goal, but Durno's goal is desperation," Sacco said. "It was a huge goal because it tied the game and gave us life."
Before their rally, the Avalanche didn't have much energy. They took four penalties in the second, and although they killed all three Atlanta power plays, Little's 10th goal of the season with 14:28 left in the period gave the Thrashers a 2-1 lead.
The Thrashers thought they had a 3-1 lead when a ruling on the ice gave them a goal with 2:08 left in the second, but a video review showed the puck didn't completely cross the goal line and it was overturned.
"I couldn't see it. I have to trust what the camera sees," Anderson said. "I know it was pretty close."
Moments after killing a Colorado power play, the Thrashers opened the scoring in the first. Antropov won a faceoff in his own zone and got the puck to Bergfors, who skated into the Avalanche zone and used a screen by Little to beat Anderson.
Colorado struck back 46 seconds later when Brett Clark beat Hedberg.
Cumiskey's goal was the fastest overtime goal in franchise history. ... Craig Anderson is the fourth goalie in franchise history to win 30 games in one season. ... Avalanche C Ryan O'Reilly returned to the lineup after missing a game because of a foot injury.
Copyright by STATS LLC and The Associated Press
Arena: Pepsi Center Location: Denver, Colorado
Referees: Wes McCauley, Brad Watson Linesmen: Mark Pare, Jay Sharrers
Attendance: 11,644 (64.7% full)
Team Stat Comparison
Power Play Goals
Power Play Opportunities
Shots on Goal
Niclas Bergfors (15)
Assist: Nik Antropov
Brett Clark (3)
Assists: Paul Stastny, Chris Stewart
Bryan Little (10)
Assists: Marty Reasoner, Johnny Oduya