Tia Day made eight straight free throws in the fourth quarter as Monmouth Academy pulled away for a 39-30 upset victory over Boothbay in the Class C South girls semifinal high school basketball game at the Augusta Civic Center on Thursday.

The third-seeded Mustangs (18-3) will take on No. 4 Old Orchard Beach on Saturday at 7:05 p.m. The Seahawks finished at 18-2.

In a close game throughout, Monmouth claimed a 7-6 lead after a quarter and a 19- 14 edge at the half. Day had 11 first-half points to pace the Mustangs, while Faith Blethen, Page Brown and Nicole Clarke each scored four points for the Seahawks.

Boothbay made its run in the third quarter, out-scoring Monmouth, 11-4. Six straight points to open the half gave the Seahawks a 20-19 lead. The Mustangs held a 23-21 lead, but Brown scored inside to tie it and gave Boothbay a 25-23 edge after three quarters late in the frame.

Monmouth’s Kaeti Butterfield put the Mustangs ahead to stay with 3:58 remaining in regulation, and Day took over at the free-throw line from there. Blethen drew Boothbay to 33-30 with 40 seconds left, but Day made her free throws as the Mustangs scored the final six points to secure the win.

Day had a game-high 20 points, while Libby Clement chipped in six.

Brown led Boothbay with 12 points, while Blethen and Clarke added eight points apiece. At the line, Monmouth made 18-of-22, while the Seahawks were 2-of-8.