SOUTH BERWICK — With the ball bouncing around the front of the field hockey cage and bodies diving all around her, Mallory Nadeau didn’t have time to think, just react.

So she stuck her stick out and knocked the ball into the cage for the goal that sparked Marshwood High to a 2-0 victory over Scarborough in the Class A South field hockey semifinals Monday afternoon.

Nadeau scored off a penalty corner 68 seconds into the second half as the second-ranked Hawks moved to a regional championship game for the third time in six years. Marshwood (14-2) will play fifth-ranked Massabesic on Wednesday at Massabesic High, the time to be determined Tuesday. The Mustangs defeated Marshwood 2-1 on Sept. 13.

“We have worked so hard,” said Elaine Bachelder, a senior midfielder for Marshwood. “I’ve dreamed about this since my freshman year.”

The Hawks, who beat Scarborough 4-1 in the season opener, had gone 0 for 10 on penalty corners in the first half, which ended in a scoreless tie. But they weren’t concerned. Instead, Coach Lisa Truesdale designed a corner play at the half. And the Hawks scored off both penalty corners they had in the second half.

On the first one, Celine Lawrence inserted the ball from the left of the goal to Hannah Costin at the top of the circle. She returned the ball to Lawrence, whose shot was stopped by Scarborough goalie Sam Carriero. But the ball bounced around, eventually ending up on the opposite side, when Nadeau poked it in with 28:52 remaining.

“Everyone was diving around,” said Nadeau, a senior forward. “I just don’t think about it, I just go for it. I just dove in. It’s all instinct.”

Marshwood’s defense – which has given up just five goals all season – allowed three shots on goalie Shannon Giblin, with Bachelder and Costin dominating the midfield.

“That’s a great team, they deserved it,” said Scarborough Coach Kerry Mariello. “Between Elaine and Costin, they’re outstanding. They balance the field very well because they’re playing opposite sides of each other. So you try to go up one end and there’s one of them and you try to go up the other end and there’s the other. Then they collapse in the middle whenever we try to go up the middle.

“They’re the ringleaders of this whole dynamic. And naturally the other girls around them are going to be better because of them and playing up to their standards.”

Costin scored with 2:49 remaining off another penalty corner, assisted by Bachelder, to make it 2-0.

“One goal, no matter how ugly you’re playing, is just one goal,” Mariello said. “And if we could have just broke through the momentum could have shifted. That second goal takes the air out of you.”

Truesdale said the victory was a credit to her team’s leadership and perseverance.

“Scarborough is a team that’s in the playoffs constantly,” she said. “So when they get into the postseason, they know how to react. For us to come in, and there was a lot of pressure on us because we’ve never been the higher seed, I was impressed with this group.”