The loss snapped a three-game losing streak for the Caps, and it was a franchise-record sixth straight victory for the Thrashers, who entered the evening 30 points behind Washington in the Southeast Division.
Colby Armstrong put the Thrashers on the board at 10:13 of the opening period. Rich Peverley weaved his way through the Caps defense with the puck, and eventually Armstrong sent it toward the net. It hit off defenseman Shaone Morrisonn and past goalie Jose Theodore.
Brian Pothier, playing in his first NHL game since Jan. 3, 2008, was assessed a hooking penalty later in the period, and Atlanta scored its first of three straight power-play markers. After a couple of blown chances to get the puck out of their end, Thrashers forward Slava Kozlov took advantage of a tired Caps penalty-killing unit at 15:43. He walked the puck from the right point into the circle and ripped a shot through a crowd for his 20th of the season.
Pothier was recalled Sunday from Hershey of the American Hockey League after a four-game conditioning stint with the Bears. He logged 16:51 of ice time, including 57 seconds on the power play. It was all Thrashers again in the second period. With John Erskine in the box, recent addition Anssi Salmela collected the puck in his own end and skated down the right wing and toward the slot before putting a shot past Theodore to the far side at 7:18.
The goal was the first for Salmela, who was acquired when the Thrashers sent defenseman Niclas Havelid to New Jersey earlier this month.
Alexander Semin took a hooking penalty just seconds after Milan Jurcina had served two minutes for boarding, and the Thrashers pushed their lead to four goals. Tobias Enstrom's shot from the top of the circle at 12:59 found its way through a screen provided by Bryan Little, and it proved to be the last shot Theodore faced. He was replaced by Simeon Varlamov, who was also recalled Sunday from Hershey and was appearing in his third NHL game.
Washington didn't have a power play in the first 37 minutes of the game. The first came when Atlanta's Boris Valabik slashed Alex Ovechkin on the hand. Ovechkin shoved Valabik after the play and proceeded to deliver three crunching hits in his next two shifts -- but it was far too late.
Eric Perrin added the final insult at 16:38 of the third when he picked off a Poti pass and then put home his rebound through Varlamov's legs.
Eric Fehr scored the Caps' only goal of the night with 1:15 left.
Ovechkin has typically feasted on Lehtonen in his career, and his 23 goals against Atlanta is the most against any franchise. But Lehtonen won the battle Tuesday night, turning aside 10 shots from last season's MVP.