Goaltender Braden Holtby watches Matt Niskanen’s game-winning goal sail by as the Penguins score moments after stoning the Capitals on a four-minute power-play. (Justin K. Aller/Getty Images)

The Washington Capitals owned the third-best power play in the NHL heading into Tuesday night’s game and had already scored once with the man advantage against their heated rival. So when Pittsburgh Penguins forward Matt Cooke received a pair of minor penalties in the third period of a tie game, the ensuing four-minute power play seemed like a gift.

From the outset, though, the Capitals’ power play was out of sorts. They struggled to maintain possession of a bouncing puck, their passes were off the mark and they stood still, making it easy for Pittsburgh’s penalty killers to limit their looks.

Washington failed to convert. The capacity crowd at Consol Energy Center rose to its feet and as the building roared it felt like the home team would inevitably score. Just nine seconds after thwarting the penalty, Pittsburgh did. Defenseman Matt Niskanen recorded the game-winner in a 2-1 triumph over the Capitals, the Penguins’ 10th consecutive win.

“Four minutes, I think we have only two shots on net. It’s awful. It kill us,” said Alex Ovechkin, who recorded one of the two shots during the power play but had another attempt miss the target and another blocked.

With the loss, the Capitals (12-16-1) remain stuck at 25 points, seven out of the eighth and final playoff spot in the Eastern Conference and nine behind the Southeast Division-leading Winnipeg Jets.

The Post Sports Live crew debates whether the Capitals can make the playoffs and, if after their upcoming week against tough teams, they will be buyers or sellers. (Post Sports Live/The Washington Post)

Until that game-altering power play in the third period, it was a tight game. The Capitals did a solid job of pressuring the Eastern Conference leader’s offensive talents, batting down passes and taking time and space away from Sidney Crosby, the NHL’s leading scorer.

The squads traded power-play goals in the second period. Ovechkin scored for Washington and less than three minutes later defenseman Paul Martin answered for Pittsburgh. With little discernible advantage for either squad as the game progressed into the third period, it was a waiting game to see what might tip the balance.

Cooke received a minor for boarding Ovechkin with 7 minutes 49 seconds gone in the third period and earned an extra minor for unsportsmanlike conduct after complaining about the call. Here was Washington’s opportunity.

“That power play was going to be the game. I think everybody knew it,” said Eric Fehr, who recorded the other of Washington’s two shots on the advantage. “You got a four-minute power play the last 10 minutes of a game. Obviously we need to score.”

The Capitals loaded up the power play but after a faceoff win by Nicklas Backstrom against Crosby, they struggled to set up. John Carlson lost an edge and nearly sent the Penguins on a short-handed rush up ice. He recovered, but the power play didn’t look comfortable.

Ovechkin missed the net and the crowd began to murmur. A clear by the Penguins and a steal that helped them shed more time off the penalty kill brought the noise in the arena to a rumble.

“I think they just give us the pressure. So we just don’t move the puck well, and if we move the puck, the guy’s still standing,” Ovechkin said. “We don’t move our bodies to find the right spot, an empty spot.”

The crowd noise continued to escalate as defenseman Deryk Engelland blocked a shot from Ovechkin and then again as Penguins goaltender Marc-Andre Fleury (28 saves) turned aside the shots by Fehr and Ovechkin five seconds apart.

“They did a pretty good job keeping us to the outside,” Fehr said. “We just passed it around, maybe trying to get a little bit too fancy. We’ve got to find a way to just have some more shots and have some traffic. They did a good job boxing out in front. I think Fleury was seeing a lot of pucks.”

As the final seconds ticked off the Capitals’ power play, the black and gold faithful stood in raucous salute of the penalty kill. Cooke exited the box with them still on their feet, seemingly willing on what was bound to come next.

“We were just yelling it on the bench. You feel that momentum switch and obviously the place is getting crazy and you’ve got to be able to withstand that,” Coach Adam Oates said. “But we turned it over at the end.”

Joel Ward turned the puck over to Niskanen when he tried to center the puck from behind the net, sending the Penguins on a three-on-two rush. Crosby carried the puck over the blue line before passing to Cooke, who found Niskanen trailing for the shot that made all the difference.