"I gave their best player a breakaway to lose the game for us," Vanek said after the Sabres were beaten by the Atlanta Thrashers 3-2 in overtime Wednesday. "It's disappointing. It's tough when you let your teammates down like that."
Kovalchuk scored on an unassisted breakaway with 29.9 seconds left in overtime to give Atlanta the win.
Vanek was near the blue line and was trying to make a pass into his team's zone, but he bounced the puck off Kovalchuk's skate.
Sabres goaltender Ryan Miller stepped to the top of the crease to try and stop Kovalchuk, whose wrister went under the goalie's legs. The goal was Kovalchuk's 41st and gave him 30 in 37 games.
"I tried to go hard to the goal and go for the No. 5 hole, and it went in," Kovalchuk said. "It's a very good win."
Buffalo had a three-game winning streak snapped. With 83 points, the Sabres are four behind ninth-place Florida and five behind Montreal and the playoff cutoff in the Eastern Conference.
Of Buffalo's final six opponents, four are among the top five teams in the East. The Sabres will also play defending Stanley Cup champion Detroit, the No. 2 team in the West.
"It's a tough point to lose," coach Lindy Ruff said. "We had the opportunity to win it. Derek [Roy] had his breakaway. [Jason] Pominville had the great opportunity off the faceoff. [Jaroslav] Spacek had another great opportunity and didn't score. If you don't finish those off, you end up losing a point."
Atlanta has won three straight and 10 of 13.
Vanek scored twice to give him 37 goals for Buffalo, which began the game with the NHL's third-best power play on the road. Those brief accomplishments were wiped out, however, by Kovalchuk.
"It was a stupid play," Vanek said. "Get it low, and give my teammates a chance to get the extra point."
Thrashers rookie defenseman Zach Bogosian's seventh goal, a wrister from the top of the right circle, forced a 1-1 tie 8:36 into the third. The NHL's third overall draft pick was one of four Thrashers penalized for roughing in the second. Officials had to separate Bogosian and Buffalo center Dominic Moore as the two exchanged shoves after the period ended.
"We are playing the spoiler role, I guess," Bogosian said. "We are trying to play our hearts out. Hopefully we can do some damage around the league. We are showing right now that we are not a team to take lightly."
Atlanta's penalty-killing unit, which ranked 29th at home, couldn't overcome a two-man disadvantage that began 12:50 into the second when Garnet Exelby went to the box for roughing. Vanek's wrister from the top of the slot 1:06 later made it 2-2.
Vanek's first goal, swept in from below the right circle, beat goalie Johan Hedberg's glove side.
Slava Kozlov was in the crease when he poked in his 25th goal to give the Thrashers a 2-1 lead midway through the second.
Miller, who dropped to 8-4-1 in his last 13 games and 31-17-5 overall, stopped 27 shots, but none of those meant much after Kovalchuk beat him.
"I tried to cheat the five-hole," Miller said. "He got it up over my stick and hit me straight in the pad right above where the knee break is and powered it through."
Hedberg, who improved to 12-10-3, made 30 saves. The Thrashers No. 1 goalie, Kari Lehtonen, has missed two straight games with an undisclosed upper-body injury.
- Atlanta is 12-3-3 against the Sabres at home, and the Thrashers have won the last five in the series -- three in shootouts and two in overtime.
- Vanek has 19 power-play goals, tied with Calgary's Michael Cammalleri for the NHL lead.
- Kovalchuk has five goals and five assists in his last five games. With 41 goals overall, he's tied with Philadelphia's Jeff Carter for third-most.