Ovechkin scored his team-high 17th goal before being tossed in the second period. Matt Pettinger tallied twice, and Ben Clymer and John Erskine also scored for the Capitals, who have won a season-high three straight.
Derek Roy, Chris Drury, Thomas Vanek and Drew Stafford had goals for Buffalo, which lost for only the second time in 13 road games. The six goals allowed were a season high for the Sabres, who had a two-game winning streak snapped.
Washington played the final 35:08 without Ovechkin, who was assessed a game misconduct for the first time in his two-year career after shoulder-checking Daniel Briere from behind into the boards near the Buffalo bench.
The Capitals scored three times in the first 7:45, sending goaltender Ryan Miller to the bench for Martin Biron. It was the third consecutive game in which Washington forced an opponent to switch goalies, an NHL first this season.
Zednik tallied on a breakaway at 2:01, pushing the puck past Miller's glove side, and Muir swept a shot from point-blank range 34 seconds later.
Ovechkin's goal made it 3-0. After stealing the puck from Maxim Afinogenov, Ovechkin kept it on a 2-on-2 break, scoring from the right wing.
Roy scored for Buffalo at 15:09 when sprawling goalie Olaf Kolzig failed to corral the puck with his gloved hand.
Less than a minute later, Clymer restored Washington's three-goal cushion. Clymer's shot from deep in the slot was chested by Biron, popped up in the air and then bounced behind him.
Early in the second, Ovechkin levelled Briere into the right-wing boards, drawing a five-minute boarding major and touching off a series of skirmishes. Paul Gaustad pummelled Ovechkin, who didn't throw a punch in retaliation.
Ovechkin, Gaustad and Buffalo's Adam Mair were handed game misconducts, and 79 minutes in penalties were assessed following the fracas.
With the teams skating 4-on-4, Washington took a 5-1 lead at 7:24. Biron came out too far to play Brian Pothier's shot and Pettinger poked the puck into an empty net.
Afinogenov's apparent power-play goal at 9:18 was disallowed on video replay when it was ruled to have been kicked in. Just 28 seconds later, Afinogenov assisted on Drury's man-up goal from in front.
Vanek made it 5-3 with 28 seconds left in the second.
Pettinger's second goal put Washington up by three. Stafford got Buffalo within 6-4 with 7:46 left.
Erskine added a short-handed, length-of-the-ice, empty-net goal with 56 seconds remaining.