FALMOUTH — With his team down two goals to Falmouth early in the second quarter, South Portland goalie Ben Kieu didn’t panic.

“You’ve just got to trust your family. This is my family,” Kieu said.

Kieu’s play in net was the fulcrum of an all-around excellent defensive effort by the Red Riots, who pushed aside a 12-goal loss to Falmouth just a month ago and took an 8-4 victory Wednesday in the Class A boys’ lacrosse semifinals.

South Portland (13-3), the No. 3 seed, advances to the Class A state championship game for the first time since 2015, where it will face No. 1 Cape Elizabeth (16-0) on Saturday at Portland’s Fitzpatrick Stadium.

No. 2 Falmouth, whose only previous losses were against Cape Elizabeth, ends the season at 13-3.

“Everyone doubted us because we lost to them by who knows how much last time,” said Kieu, who finished with 21 saves Wednesday. “We weren’t really ready for the first game. Everyone was nervous.”

The Navigators won 19-7 at South Portland on May 13. That loss exposed the Red Riots’ team defense, said South Portland Coach Dan Hanley. Improving it became a focal point, and Hanley credited defensive coach Sam MacKenzie for his work in tuning up the unit. Seniors Finn O’Donnell and Brady Angell, along with junior Jamen Senter, led South Portland’s defensive effort.

“We depended a lot on 1-on-1 defense and guys just winning their matchups. (Falmouth) has some great 1-on-1 dodgers, and we didn’t have our help defense locked in. We really practiced that a lot,” Hanley said. “We lost to this team by 12 goals the first time around. We felt if we were tight, we were in it. I don’t think we overthought it today.”

Giovanni Guerette’s goal with 9:17 left in the second quarter gave Falmouth a 3-1 lead. But the Red Riots scored the next seven goals while holding the Navigators scoreless for 24 minutes of game action. When Guerrette scored with 7:22 left, it only cut South Portland’s lead to 8-4. The Navigators had plenty of chances, but Kieu made stop after stop.

“He’s our rock. He hasn’t had to do that a lot this year, but we know he has it in him. He’s the best goalie in the state. He’s a dominant player, and you saw that today,” Hanley said.

Falmouth Coach Dave Barton said the Red Riots did a nice job clamping down on his team’s offensive depth.

“We had things we were looking to exploit. Ben stood on his head at times, but there were plenty of times we couldn’t generate the type of offense we wanted to,” Barton said.

Beckett Mehlhorn and Lucas Mehlhorn each scored three goals for the Riots, while Jack Dreifus had two assists.