PORTLAND — Indoor tennis in late May always carries with it an antiseptic air, absent sun and wind and noise from passing traffic.

On Monday afternoon inside the Racket & Fitness Center, yet another element was missing: partisanship.

Both girls and boys championship matches of the Maine Principals’ Association singles tennis tournament pitted teammates against each other, including, for the first time, siblings.

“No one knows who to root for,” said Falmouth senior Alex Klemperer, who dropped a 6-4, 6-1 decision to Falmouth junior Nick Forester in the boys final. “All our teammates are here, but we’re both on the same team.”

Klemperer spoke in a whisper because the girls championship match on an adjacent court was still being contested.

“Those are literally sisters,” he said of Grace and Rosemary Campanella. “You can’t choose a favorite.”

Grace, a sophomore at Kennebunk High, and Rosemary, a senior at Wells, play on the same coop team. On Monday afternoon, youth prevailed as second-seeded Grace defeated top-seeded Rosemary 6-2, 6-4.

Both matches ended in warm embraces at net. For the elder Campanella, Monday also marked her third appearance in the state finals.

“It’s totally different than playing anyone else,” Rosemary said quietly, seated courtside after the match. “She’s the one person that I don’t want to beat.”

Not since 2010 had teammates met in the singles final. Eight years ago, Brandon Thompson of Waynflete beat Patrick Ordway, also of Waynflete, and Ordway went on to win the following year.

“It was obviously a lot different than last year,” said Forester, who came up short in a hotly-contested 3-6, 6-4, 7-5 final to Nick Mathieu of Mt. Ararat last spring. “It was fun playing a teammate. We’re good friends and we play all the time.”

Monday marked the fourth consecutive spring in which inclement weather forced the final chapters of the singles tournament indoors. Originally scheduled for the Wallach Tennis Center on the campus of Bates College in Lewiston, Monday’s action moved inside because of early-morning rain.

In morning semifinal matches, Rosemary Campanella advanced 6-0, 6-0 over No. 12 Jocelyn Bolt, a sophomore from Gorham High while Grace Campanella defeated sixth-seeded Caitlin Cass, 6-1, 6-1. Cass is a freshman from Lincoln Academy in Newcastle who recently turned 14.

Bolt reached the semis by upsetting two higher seeds on Saturday. She knocked off No. 5 Meredith Kelley of Falmouth 6-7 (4-7), 6-4, 6-4 in the Round of 16 and No. 4 Gabrielle Marquis of Caribou 6-3, 6-1 in the quarterfinals.

In one boys semifinal, Forester dispatched No. 4 Peter Mao, a senior from Mt. Ararat, 6-1, 6-1. “The score wasn’t close but all the games were,” Forester said. “The first four or five went to deuce.”

In the other semifinal, the third-seeded Klemperer needed two hours to hold off No. 7 Jacob Greene, a Waynflete senior, 4-6, 6-4, 6-3. Like Forester, Greene hits forehands from either wing.

“I feel pretty good,” said Greene, who knocked off No. 2 Declan Archer of Kennebunk 6-2, 4-6, 6-4 in Saturday afternoon’s quarterfinal round. “I thought I played well throughout the tournament.”

The opening set of the boys final was tied at 3-all before Forester broke serve and then held twice more to close it out at 6-4. The second set was a bit more lopsided.

“I’m toast,” Klemperer said after the finals. “We were whaling on the ball, so it was a good time. I’m just happy that Falmouth is bringing it home this year. Nick definitely deserves it.”

“That first set was really tight,” Forester said. “He was putting so much pressure on me with how deep he was hitting the ball and how hard he was hitting the ball. It was tough to find a rhythm against him the first set.”

In the girls final, Grace Campanella jumped to a 4-0 lead in the opening set and rallied after falling behind 3-2 in the second. She had won a challenge match against Rosemary in pre-season but never played No. 1 for Kennebunk/Wells because of a back injury before the team’s opening match.

After rehabilitation and six missed matches, Grace returned and lost a required challenge match (an 8-3 pro set) to Rosemary, who will now play No. 2 singles in the team tournament that begins this week.

“We practice together a lot but we try not to play matches against each other because we’re really competitive,” said Grace, unlike her sister a left-hander. “At the court at home we’ll try to play a couple games or a set and it never goes well.”

Jacqui Holmes, the second-year coach of Kennebunk/Wells, said the match was hard to watch.

“I know that they don’t enjoy playing each other,” Holmes said. “I can’t imagine what they’re feeling. But it also is fantastic tennis. It’s a pleasure to watch because it’s the best tennis that we’ve seen all year.”

Preliminary-round matches for the MPA team tournaments begin Tuesday.

“Now the focus shifts,” Forester said, “to winning states as a team.”