The Mystics begin to celebrate their series-clinching Game 5 victory over Atlanta on Tuesday. (Katherine Frey/The Washington Post)

Elena Delle Donne sat on a chair in the Washington Mystics’ locker room late Tuesday night at McCamish Pavilion, her left knee wrapped in ice to limit swelling from a bone bruise that, by all accounts, would have kept most players out of the lineup.

She wasn’t about to be only a spectator in the most important game in franchise history, so Delle Donne had slipped on a protective brace and proceeded to play more than 36 minutes in a winner-take-all Game 5 against the Atlanta Dream to send the Mystics to the WNBA Finals for the first time.

That she pushed through the discomfort to collect 14 points and 11 rebounds left her teammates marveling at her moxie, but Delle Donne counted the performance as simply part of her promise from the day she arrived to do whatever necessary to deliver the Mystics a championship.

Washington now stands within reach of its first professional basketball title since the Bullets won the NBA Finals in 1978 in the same city where the No. 3 seed Mystics headed Wednesday afternoon — Seattle. They are preparing for the best-of-five series that begins Friday night against the top-seeded Storm.

“I’m just so proud of this group,” Delle Donne said. “We’ve got amazing people in this locker room. All season long, it’s been team, team, team. It’s about we, and that’s what got it done, and that’s what’s going to continue to get it done.”

The Mystics’ record holder for single-season scoring average is making her second trip to the WNBA Finals and first since 2014 when she played for the Chicago Sky.

Coach Mike Thibault, meanwhile, is back in the Finals for the first time since 2004 and ’05, when he coached the Connecticut Sun. Neither Delle Donne, a five-time all-star and the 2015 league MVP, nor Thibault, the league’s all-time leader in coaching victories, have won a title.

The only player on the Mystics with a WNBA championship ring is guard Kristi Toliver. The two-time all-star won hers with the Los Angeles Sparks in 2016, beating the Minnesota Lynx in five games.

“Don’t make the moment bigger than what it is,” Toliver said of how the Mystics should approach their first Finals appearance. “Enjoy each moment but be confident in yourself and be confident in who we are as a team. I think belief is such a huge thing, especially going into a Finals situation. You have to trust and believe that you’re capable of winning it, and I think that we are.”

The Mystics lost twice in three regular season games this year against Seattle, falling both times on the road, but Thibault cautioned not to draw too many conclusions based on those circumstances.

In the first matchup, May 29 at KeyArena, Washington was without Delle Donne and fellow starters LaToya Sanders and Natasha Cloud but kept the margin close in an 81-77 loss. It was the first of four consecutive games Delle Donne missed because of a recurrence of Lyme disease. Sanders was recovering from anemia, and Cloud had a kidney issue.

The teams played again July 8, with the Mystics on the second leg of a back-to-back and playing three games in four days. They didn’t arrive at their hotel until early on the morning of the game but were within several possessions down the stretch in a 97-91 loss. Delle Donne finished with 29 points, seven rebounds and four assists.

“I don’t know that either team has a really good read on the other team,” Thibault said, “other than we play really similar.”

He pointed to several matchups in particular, beginning with Delle Donne and Storm forward Breanna Stewart, this year’s league MVP. Then there’s Toliver and veteran point guard Sue Bird, who led Seattle to a 94-84 win over the Phoenix Mercury in its decisive Game 5 late Tuesday night. Bird, who was playing with a broken nose, scored 14 of her 22 points in the fourth quarter.

Bird made 5 of 6 shots over the final six minutes, helping the Storm rally from an 11-point deficit in the first half to reach the WNBA Finals for the third time in franchise history. Seattle won league titles in 2004 and 2010 with Bird on both teams.

“The Finals bring out the best in people,” Toliver said. “We have a lot of good people. They have a lot of good people. I think you can tell by the way the regular season went that we match up pretty well with them, and they match up pretty well with us, so I’m excited about it. I’m excited to face Sue, Stewie and Jewell [Loyd], friends and great competitors.”