Nate Glidden converted a tough layup with 1.8 seconds left in overtime to provide the winning points as No. 6 Scarborough posted the first major upset of the Maine high school basketball tournament with a 59-58 win against No. 3 Gorham in a Class AA South boys’basketball quarterfinal at a loud and packed gym at Gorham High.

Scarborough entered the tournament with a 6-12 record and had lost twice to Gorham by double-digit margins in the regular season. The Red Storm (7-12) will face the winner of Wednesday’s quarterfinal between No. 7 Sanford and No. 2 South Portland in the semifinals at 7 p.m. Feb. 23 at Cross Insurance Arena.

“For the past week I’ve told (my teammates) just ‘Believe, believe, believe,’” Glidden said. “I told these guys, the season didn’t go the way we wanted but … I know who we are, we knew who we were inside, and we came out here today and really proved it.”

De’Angelo Alston led all scorers with 22 points for Scarborough and made two clutch free throws with 1.9 seconds left in regulation to force overtime. Liam Garriepy added 14 before fouling out early in the fourth quarter.

Gabe Michaud scored 17 points for Gorham (13-6) with Ashton Leclerc (16) and Devin Dobson-Jacques (13) also hitting double figures.