Natasha Cloud leads the locker room celebration after the Mystics advanced to the WNBA Finals for the first time in franchise history. (Katherine Frey/The Washington Post)

With ailing forward Elena Delle Donne having a relatively modest shooting night, at least by her lofty standards, the Washington Mystics instead leaned on a rookie in the decisive Game 5 of the WNBA semifinals Tuesday against the Atlanta Dream.

Ariel Atkins delivered with a masterful performance belying her youth, collecting a game-high 20 points in addition to seven rebounds and three assists as the Mystics secured an 86-81 triumph at McCamish Pavilion to advance to the WNBA Finals for the first time.

Washington will face the Seattle Storm, which eliminated the Phoenix Mercury with a 94-84 win in Game 5 of the other semifinal series Tuesday. Game 1 of the Finals is set for Friday night in Seattle.

In the finale against the Dream, the Mystics also got 19 points from guard Kristi Toliver and 17 points from reserve forward Tianna Hawkins to rally from a 2-1 series deficit. But it was Atkins who stood out in some of the most critical junctures, including starting a decisive run midway through the fourth quarter that delivered the Mystics the lead for good.

“That was one of the most clutch performances I’ve ever witnessed,” Delle Donne said of the No. 7 overall pick in April’s draft. “And then to be like, ‘Also, she’s a rookie,’ That’s crazy.”

Atkins scored 12 points in the second half, including four important free throws in the closing minutes. In one stretch in the third quarter, she either scored or assisted on eight consecutive points that put the Mystics ahead by two.

“She’s abut the most poised rookie I’ve ever been around,” Coach and General Manager Mike Thibault said.

Trailing by two with 5:43 left in the fourth quarter, the Mystics scored nine consecutive points to claim a 78-71 lead with 3:48 to play. Even though Delle Donne didn’t make a field goal in that surge, she contributed a steal, a block and an offensive rebound before Tiffany Hayes ended Atlanta’s scoring drought.

But Delle Donne got a difficult up-and-under jumper to fall, providing Washington with an 80-73 lead with 1:41 left. Hayes answered again, this time with a three-point play.

But Atkins made two free throws soon after, and Delle Donne closed the game with four straight foul shots to secure just the second playoff series win in franchise history. Delle Donne finished with 14 points on 5-for-13 shooting with 11 rebounds and four assists.

“Ariel’s capable of doing that whenever she wants to,” Toliver said. “It is mind-set, and I think this team, we have so many threats. Again, when it’s win or go home, you have to elevate yourself, be the best version of yourself. [Delle Donne], she’ll be better. She’s going to get herself back. It’s amazing that she’s even playing right now.”

Delle Donne has made time to mentor Atkins despite dealing with bone bruise in her left knee suffered in Game 2 in Atlanta.

The injury odyssey for Delle Donne began with just over three minutes left in the fourth quarter of that game. While dribbling to the basket, Delle Donne planted her left foot and her knee bent awkwardly. She immediately collapsed beyond the baseline next to the Dream bench, clutching her left knee visibly in distress.

An MRI exam the following morning in the District revealed a bone bruise but no ligament damage, as initially feared. Delle Donne began undergoing extensive treatment, and after sitting out a Game 3 loss at Smith Center, she started Game 4 with the Mystics on the verge of elimination.

After testing out her knee in warmups before Game 5 and wearing a protective brace, Delle Donne showed little if any indication of discomfort in the early going, first coming to a jump-stop with her left leg while attempting a fadeaway and later in the first quarter driving aggressively to the rim for a jumper off glass.

Washington led by eight points in the first quarter, but the Dream rallied with nine consecutive points to move in front by one. The teams finished the quarter tied at 24 after guard Renee Montgomery was fouled on a three-point attempt and made all of her free throws.

Toliver missed a layup at the buzzer but became the Mystics’ best scoring option in the second quarter, making a pair of three-pointers during a stretch in which she had 10 of her team’s 12 points.

The first half concluded with the Dream leading 50-49 on the heels of six points in a row to close the second quarter, but the Mystics did enough down the stretch to reach new heights in the franchise’s 21-year history.

“It’s been a long time coming for the organization,” said forward Monique Currie, a 13-year WNBA veteran and District native who spent 2008 through 2014 with the Mystics before coming back this season. “I’ve been around for a long time here, like growing up a fan of the Mystics and then being a member of the team. It’s just so awesome.”