And now, there’s tension.

A joyous, breezy Washington Capitals season finally succumbed to the playoff strain Friday night at Verizon Center. Leading Philadelphia three games to one in this first-round Stanley Cup playoff series, the best team in hockey returned home for a closeout game, turned in a performance worthy of series-clinching victory and somehow still lost. The Capitals lost in a most mystifying and unreasonable manner, a cruel result that hockey sometimes delivers at random.

For a franchise that has a long and painful history of struggling in these moments, this was a torturous way to apply pressure. It’s one thing to choke. It’s another to be tagged by bad luck. After a star-crossed 2-0 loss to the Flyers in Game 5, what was a 3-0 series lead has melted down to 3-2. A potential short series is now a long and taxing one. On Sunday, the Flyers return to Philadelphia for Game 6 with a chance to tie the series and extend it to a full seven games. You can feel Washington’s pressure swelling.

If you could attach words to bewildered expressions, they would be terse and emotional.

How? Why?

Not again.

And, of course, the nail-gnawer: Uh-oh.

During Alex Ovechkin’s 11 seasons in D.C., the Capitals have had six postseason series leads of at least two games. They’ve gone on to win the series just once so far.

Yeah, this series just got real.

The Capitals aren’t just competing against the Flyers and the high standard they set this season anymore. They’re nose-to-nose with their demons, too.

“I don’t know if it’s fair or not,” Capitals Coach Barry Trotz said of the constant references to the Capitals’ playoff history. “It doesn’t really matter. Everybody always talks about the past, the past, the past. The only pressure we really have is on ourselves.”

The intrigue begins with how the Capitals digest this game. They played well. They dominated the Flyers, creating 33 more shots on goal. The 44-11 advantage equates to quadruple the opportunities for Washington. The Capitals competed with great energy the entire game. They were more aggressive, more physical, superior in every way. Philadelphia’s only true goal, from Ryan White, came after the puck ricocheted off Capitals defenseman Taylor Chorney’s skate. The Flyers’ second goal, by Chris VandeVelde, was an empty-netter in the final minute.

The Capitals were the beneficiary of a ridiculous 101-foot goal in Game 2. In Game 5, White’s goal equalized the luck. And the desperate team is feeling bolder.

“We’ve been playing Game 7 every night,” White said.

Flyers goaltender Michal Neuvirth, who replaced Steve Mason in Game 4, has changed the series. He has stopped 75 of 76 shots over the past two games, including a playoff-career-high 44 saves on Friday night against his former team. Combine Neuvirth’s magnificence with the Capitals’ bouts of undisciplined play — they committed eight penalties and spent 19 minutes trying to kill off disadvantages — and an inexplicable loss becomes easier to understand. The Capitals had so many opportunities, but they weren’t as fluid as they’ve been. With Neuvirth at his best, sprawling to deny Karl Alzner, corralling a slap shot from Ovechkin and snuffing out Dmitry Orlov on a rush, the Capitals paid for even the slightest inaccuracy.

So another big playoff lead is whittling, but this team was built to handle the situation better. Still, history hovers. Is this team different? Or did Friday night provide reason to fret?

After a poor effort in Game 4, the Capitals played with greater urgency this time. But to Trotz, the bad bounces of Game 5 served as punishment for not making a better attempt to close the series in the previous game.

“I always talk about the hockey gods,” Trotz said. “People think I’m crazy. Game 4, I didn’t think we played hard enough in that game. . . . What we’re learning is we’ve got a resilient team on the other side. It doesn’t matter how they get [a goal], but they got one. And we didn’t get any.”

Still, if the Capitals bring their Game 5 intensity to Philadelphia, clean up a few things and play a little more disciplined, they’re still in control of this series. It’s unlikely they could play two consecutive games at this level and lose both. But is it possible to carry over the momentum of a performance that resulted in a loss? They’ll be forced to find out.

“I thought tonight we played excellent,” Trotz said. “What are you going to say? Keep playing that way, and it’s going to turn.”

This was the game the Capitals had to win. This was the game that a team of their caliber typically wins. After taking a 3-0 series lead, they failed to eliminate Philadelphia in Game 4 on Wednesday night, but they were back home for Game 5. No more complacency. It was time to show a killer instinct that other Washington squads haven’t had. It was time to advance to the second round.

From the start, they were intent on doing just that. The game began with a fight between T.J. Oshie and Brayden Schenn right after the puck dropped. For Oshie, it was payback for Schenn’s dirty cross-check on Evgeny Kuznetsov in Game 4. It triggered a physical Washington style that remained the entire night. The Capitals had an aggressive start and outshot Philadelphia 14-6 in the first period and outhit them 11-5.

In the second period, the Capitals looked even more dominant. They had 16 shots on goal to the Flyers’ two. But White’s goal bounced off Chorney’s foot, and the stress of this playoff series kept multiplying for Washington.

It was once 3-0. Now it’s a series. And the Capitals don’t have a track record of composure on their side.