SCARBOROUGH — Rachael Wallace’s precisely placed goal through a screen with 10:48 left in the first half proved enough for top-rated Scarborough High to edge No. 5 Marshwood, 1-0, in Western A regional semifinal action, Saturday.

“I saw the opportunity, shot, hoped for the best and aimed for the corner and it went in,” said Wallace, one of just three senior Scarborough starters.

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Scarborough (15-0-1) will face No. 2 Massabesic in Tuesday’s regional final at Thornton Academy. The Mustangs advanced with a 2-0 home win against No. 3 Falmouth.

Game time for the regional final will is not yet determined.

Wallace’s goal came shortly after Scarborough had reversed a 12-minute stretch of control by Marshwood (13-4).

In the second half Marshwood put four shots on goal against Scarborough keeper Alyssa Souza. Marshwood goalie Cassidy Smith — who had stopped two shots in the sequence that led to Wallace’s goal — made five second-half saves, including two sprawling stops against Abby Walker.

Scarborough has not lost to a Western A team since 2011. The Red Storm are 32-1-1 overall in that stretch, losing only to Skowhegan in the 2012 state final.

In an early regular-season meeting this year, Scarborough beat Massabesic, 5-1.