Once again, the Senators couldn't hold a lead as goalie Martin Gerber continued to struggle as he fell to 2-6-1 while stopping only 26 of 31 shots. The Senators blew 3-1 and 4-2 advantages. It was the third straight game that Ottawa blew a lead.
The Senators, who last lost five straight games from Feb. 22-March 9, 1996, led 2-0 in a 3-2 loss to Carolina on Saturday night and couldn't hold a 3-0 lead in a 4-3 loss to Washington in overtime on Monday night.
The Thrashers pulled even at 4-4 on Ilya Kovalchuk's wrist shot past Gerber about four minutes into the third period. Kozlov's third goal of the game about 12 minutes into the period gave Atlanta its first lead.
Ottawa coach Bryan Murray planned to give Gerber a rest against Atlanta, but backup goalie Ray Emery suffered a sprained wrist in practice Wednesday.
The Thrashers, who lead the Southeast Division, won their fourth straight game despite changing goalies midway through the game.
Johan Hedberg, who had allowed only 2.01 goals per game in three straight wins, was pulled after Ottawa took a 4-2 lead midway through the second period. Hedberg stopped 15 of 19 shots. Keri Lehtonen came on in relief and stopped all 19 shots he faced.
The Thrashers were called for four first-period penalties, providing the Senators repeated opportunities to shake out of their power-play slump.
Ottawa was 0-for-16 on the power play in its last three games. The Senators failed on two power-play opportunities against Atlanta before Daniel Alfredsson broke the slump with a goal for a 2-1 lead.
Chris Kelly's tip-in about 6 minutes into the second period pushed the Senators' lead to 3-1.
Ottawa's first penalty, a holding call on Anton Volchenkov, came 7:36 into the period, and less than a minute later a delay call against Andrej Meszaros gave Atlanta a two-man advantage.
Only 14 seconds later, Kozlov, positioned in front of the net, tipped in a pass from Marian Hossa to cut Ottawa's lead to 3-2.
Antoine Vermette's short-handed goal at the end of Atlanta's power play gave Ottawa a 4-2 lead, prompting Thrashers coach Bob Hartley to change goalies.
Kozlov's second goal of the second period cut Ottawa's lead to 4-3.
The Senators opened the scoring in the first period with Jason Spezza's sixth goal. Atlanta answered with a short-handed breakaway goal by Brad Larsen for a 1-1 tie.
Notes: Emery was in uniform one day after suffering a sprained wrist in practice. ... Kozlov recorded his fifth career hat trick. ... Hossa had three assists.