When the ball left Kelsey Plum’s fingers and the final buzzer went off — with the outcome still in the balance — it was official: These WNBA playoffs would be unlike both the Washington Mystics’ regular season and their run to last year’s Finals. There are matters both practical and logistical that make those things true, and they were central to the postseason opener for a team that considers a championship as the only acceptable result.

The playoffs are flat-out harder than what was a cakewalk of a regular season for the Mystics, and Plum’s last-second heave that could have forced overtime was just one heart-stopping reminder.

“Stressful,” said Mystics star Elena Delle Donne, who missed what could have been a clinching shot and then guarded Plum on the running heave that didn’t fall. All of that opened the best-of-five semifinal series against the Las Vegas Aces with a harrowing 97-95 victory for Washington that had just about everything, including the ingredient the Mystics’ regular season romp didn’t: drama.

As important, though, as the result of that shot was the building in which Plum took it — Entertainment and Sports Arena in Southeast Washington. It is a stage and an advantage the Mystics had no idea was possible a year ago.

So, consider the ingredients. These Mystics have swagger. They have skill. They have the searing experience of reaching the Finals and not winning the title. And they have a home.

That’s quite a combination. Yet the way in which the Mystics began their run to what they hope will be the franchise’s first trophy wasn’t how they have become accustomed to playing. This was difficult, in all the right ways. The favorites were pressed, and hard. They responded, buoyed not only by Emma Meesseman’s 27 points and 10 rebounds and Delle Donne’s all-around excellence — including a key bucket with about half a minute left — but by a crowd that recognizes what could be coming over the next few weeks.

“We’ve gotten to a position where we’re No. 1 and everyone’s looking to come at us,” Delle Donne said. “But that’s where we want to be.”

They are there, with reason. Put aside the anxiety of the final moments Tuesday and think about how the Mystics entered these playoffs, not just as the top seed, not just as winners of 17 of their past 19 games, not just on a 10-game home winning streak. No, they were throttling teams and stepping on necks.

Since mid-July, when that 19-game finishing kick started, they had won 13 times by double digits. Their 26 victories over the course of the season came by an average of — get this — 19.5 points. They took the mantle of favorites — a status gained in part because of their run to the Finals last season, where they lost to Seattle — and gave it a big ol’ hug.

“They had a target on their backs since the first game of the season,” Coach Mike Thibault said. “We’ve been a favorite or one of the co-favorites for a long time. And yet we’ve kept winning.”

That’s admirable. It also means nothing right about now. Go up 13 in the fourth quarter, as Washington did Tuesday, and a playoff opponent is likely to have a shot to tie or win it at the buzzer, as Las Vegas did.

“It’s playoff time now,” Delle Donne said, “so there probably won’t be blowouts.”

The first game wasn’t, and Plum’s leaner at the buzzer not only made that point obvious but was the exclamation point on so many harrowing moments that led to it. Start with Plum’s buzzer-beating jumper at the end of the second quarter, the shot that put Las Vegas up seven. The crowd of 3,968, so enthusiastic for so much of the night, fell quiet.

Wait, about that crowd. This is the Mystics’ crowd, because this is the Mystics’ arena, a brand new one to boot. That mattered throughout the season, when Washington dominated at home, and it mattered Tuesday night.

Yes, a ridiculous 8:30 p.m. tip-off time on a school night — hello, ESPN! — meant the game wasn’t a sellout. But considering where the Mystics were last fall, literally meandering to the Finals, this was a comforting feeling. The team’s old home court was cavernous Capital One Arena — too big, too impersonal, too (shhhhhhh) quiet. Kicked out of that building as it underwent renovations, the Mystics played at George Washington University’s Smith Center, then finished out the finals against Seattle at George Mason’s EagleBank Arena, out in the Virginia ’burbs.

There was, all along the way, a following. But the Mystics believe having a true home influenced their season.

“Being [14-3] in this building says a lot about what it’s given us,” Thibault said. “At the start, it’s still our skill level that has to be good to do that. But the fans have had a direct influence on the game. It energizes the team from the time you walk into the building.”

It energized the Mystics at halftime Tuesday. They opened the third quarter by scoring the first seven points, erasing the Las Vegas lead. When guard Natasha Cloud celebrated a bucket by flexing and yelling, it wasn’t into the abyss. It was to the throngs along the baseline — including Wizards guard John Wall — who screamed along with her.

From there, the Mystics pushed the lead to 13 in the fourth quarter. When Las Vegas cut it to two, they found a way. Not one of them. All of them.

“I just feel like they have a confidence about themselves because they trust each other to do their jobs,” Thibault said. “They trust each other to make shots. I don’t think any one player on this team feels the pressure to win it all by themselves.”

They didn’t Tuesday. Meesseman was central, and everything for Washington runs through Delle Donne, the MVP candidate, who added 24 points, six rebounds and six assists. But watch what LaToya Sanders did against a hulking Las Vegas lineup, scrapping for nine points, five boards and five steals. Veteran guard Kristi Toliver eased herself back into the rotation from injury and looked uneasy — but buried a massive three-pointer late in the third.

But it took Delle Donne’s bucket with 32.9 seconds left to provide the Mystics’ last points. And then it took watching Plum’s final shot.

“Wow” was Thibault’s from-the-gut reaction. “Sometimes it’s better just to survive.”

The regular season is over, and the string of blowouts might be, too. That’s okay. The Mystics — and their very own home crowd — are ready for whatever’s next.

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