The Thrashers, with 88 points, are six up on Tampa Bay, which lost 5-1 at Washington, in the Southeast Division. Buffalo, the best team in the Eastern Conference, had its two-game winning streak snapped.
Since the Thrashers traded for them in late February, Tkachuk and defenseman Alexei Zhitnik have helped Atlanta go 8-2. Zhitnik's seventh goal, coming on a power-play slap shot midway through the third, gave the Thrashers a 3-2 lead.
Buffalo tied it at 3-all on Derek Roy's 15th goal with 7:35 remaining. The puck glanced off Kari Lehtonen's mask, apparently hitting him near the right ear. After Roy lifted a wrist shot over his shoulder from the bottom of the left circle, Lehtonen fell face first to the ice.
Lehtonen, who improved to 32-21-8, stayed in the game after a trainer attended to him. He stopped 35 of 38 shots, including four in overtime, one of them a glove save to help kill a slashing penalty on Tkachuk.
Tkachuk scored his 26th goal, his sixth with Atlanta. Zhitnik, who had the game-winner in overtime on Friday against the New York Rangers, had an assist on Tkachuk's goal.
Ilya Kovalchuk's 38th goal made it 1-0 for Atlanta with a two-man advantage early in the first. Drew Stafford responded for Buffalo with his ninth goal at the 6:11 mark of the period.
Eric Belanger put the Thrashers up 2-1 on a breakaway 15:36 into the first, but Thomas Vanek's 35th goal made it 2-2 early in the second.