Potomac School celebrates its Independent School League AA tournament championship. (Gabe Hiatt for The Washington Post)

With one out separating Potomac School from winning the Independent School League AA tournament championship Sunday, junior third baseman Anna Regan surprised herself.

National Cathedral cleanup hitter Katie Johnsen smacked a ground ball down the third base line that Regan stabbed with her backhand before making a strong throw to first for the final out of a 6-4 Panthers win at Flint Hill in Oakton.

“That was probably the first backhand that I’ve made all season,” Regan said. “I was really surprised that I made that.”

The play quashed any hope National Cathedral (13-1) had for a seventh-inning rally. Junior pitcher Sarah Ing led off the final inning with a triple for the Eagles, and senior shortstop Kinza Baad’s sacrifice fly cut the lead to two, but Regan’s play prevented further damage as Potomac School (11-4) handed National Cathedral its first loss.

“It’s amazing,” Potomac School Coach Wayne Loving said. “We peaked at the right time.”

Emma Wright hold back on a ball. (Gabe Hiatt for The Washington Post)

Left fielder Katie Schluntz started the scoring in the second inning, singling and eventually coming around on a wild pitch.

National Cathedral came back to take the lead in the third on a run-scoring double by Johnsen and an RBI single by second baseman Maddy Belt.

The Panthers came right back in the fourth. Tessa Smalley tripled, then scored on an error that allowed Schluntz to reach. Catcher Bryton Fett singled to score Schluntz and help Potomac School retake the lead.

“I know we have a team with a lot of heart,” Regan said. “I knew we would do well. I had no idea that we would go this far and make such great plays. . . . We wanted this more than anything. It was our time.

Potomac School would not have reached the title game without 10th inning heroics from senior center fielder Emma Wright the previous day. Her home run Saturday completed a 3-2 win over defending champion St. Stephen’s/St. Agnes.

Wright told her teammates before the game to focus on upsetting National Cathedral, who won the regular season championship and beat the Panthers 17-7 on April 5.

“We’re definitely coming in unexpectedly up here,” Wright said. “We had it in our favor that they weren’t expecting us to be as strong as we were. We just tried to come out and surprise them.”

Potomac School captured its third ISL AA tournament title, but its first since 2005. The Panthers will finish their season next week in the Virginia Independent Schools tournament.

Holton-Arms wins A: Top-seeded Holton-Arms defeated Episcopal, 10-0, to take home the ISL A tournament championship. The Bethesda school advanced to the finals in last year’s tournament before falling to eventual champion, National Cathedral.

The win will send the Panthers up to the AA division next season.