The Washington Capitals wanted a challenge, a test by which they could grade themselves nine games into this shortened season. They got their wish Sunday afternoon against the Pittsburgh Penguins, but failed the exam.

Unlike so many previous iterations of this rivalry, there would be no real back-and-forth battle between these teams, no dueling superstars looking to one-up the other. No, this time all the usual raucous energy at Verizon Center quickly dissolved after the first period as it became clear that, for the moment, one team stands above the other as a more complete, cohesive unit.

The Capitals fell, 6-3, to Pittsburgh in a matinee meeting in which it wasn’t Alex Ovechkin (one assist), Sidney Crosby (three assists) or Evgeni Malkin (two assists) who tipped the balance, but rather the visitors’ overall superiority at taking advantage of mistakes.

Penguins winger Chris Kunitz recorded a hat trick, but his most damaging goal was his first, which gave the Penguins a two-goal lead just 37 seconds after they had broken a tie at 2. The snap shot at 7 minutes 33 seconds of the second period beat goaltender Braden Holtby cleanly and, according to multiple players, knocked the Capitals off their game.

“I think the biggest thing is just our mental game right now isn’t strong enough,” said Holtby, who was unsteady at times in his first career appearance against the Penguins and gave up six goals on 26 shots. “We’re playing a good team game. It’s just those little breakdowns, and like myself on that third goal, it’s just — those are the little things that we have to prepare for before the game that are huge in winning games.”

The Post Sports Live crew wonder whether there is a direct correlation between Alex Ovechkin’s individual goal-scoring and the Capitals’ wins and losses. (Post Sports Live)

The Capitals tied the game at 2 just more than four minutes into the second period on a fluke goal by John Carlson, whose dump-in ricocheted off the stanchion and into an open net when Penguins netminder Tomas Vokoun (21 saves) went behind the goal to play the puck.

But after a Washington power play came and went with just one shot on goal, the Penguins pushed back. Defenseman Kris Letang made it 3-2 at 6:56 of the second on a shot that fluttered under Holtby’s left arm. Kunitz followed shortly after on a rush up ice after the Capitals lost a faceoff in the offensive zone. The rapid-fire tallies siphoned the energy out of the sellout crowd and Capitals players themselves.

“That’s something that we haven’t really gotten over — when the chips aren’t quite falling our way we kind of — I don’t want to say give up, but we change our game plan a tiny bit,” Capitals forward Joey Crabb said. “We just got to stick to what we’re doing.”

When the Capitals (2-6-1) find themselves in trouble, they tend to ad-lib. Rather than rely on their system, players start to work individually, aiming for chances off the rush rather than prolonged cycles in the offensive zone. Mistakes can compound and lead to more opportunities for an opponent. It’s a dangerous habit against any team, let alone the talented Penguins (6-3-0).

“They got two quick goals, and it kind of deflated us and we weren’t able to recover,” said forward Troy Brouwer, who emphasized the Capitals need to trust their system. “I think we’ve got guys trying to do too much right now because of the situation that we’re in. Everyone wants to win it for themselves.

“You know, we’ve got to fall back on our systems and our plan as our safety net rather than trying to create things that aren’t there, because that’s when we get into trouble and that’s when we give up on odd-man rushes.”

While players cited an inability to shake off the two quick goals, first-year Coach Adam Oates disagreed.

“I don’t see it that way. I don’t. I thought we played a good hockey game,” Oates said. “We played a very good hockey game. We did what we were supposed to. They didn’t generate anything we didn’t give them.”

Kunitz added a second goal on a power play to make it 5-2 with seven minutes remaining in the second period. After Karl Alzner lost his stick, Malkin isolated the vulnerable blue-liner and sent a smooth pass across the slot to Kunitz. It added to the rough start to the season for the Capitals’ penalty kill, which has been successful on just 32 of 44 opponents’ chances.

Despite facing a three-goal deficit, Washington came out with plenty of energy in the third.

“I think if we play that kind of game like we play in the third period all game, I think we win the game,” Ovechkin said. “But first period was very tight and second we lost it, so it make a difference.”

Mike Ribeiro scored on the power play less than four minutes into the third to make it 5-3, but a Capitals comeback was never in the cards. Kunitz added a third tally with eight seconds left.

“I think it’s deflating overall when you go into a hole and then you start playing good and you see what you can do out there and just didn’t do it all game,” Carlson said. “That’s been our downfall, I think. Besides maybe one game this year, we’ve had stretches of great hockey and we obviously need to work on that because it’s not good enough otherwise.”