SACO — The Thornton Academy boys’ lacrosse team had a pretty simple assignment: Don’t lose to South Portland in the playoffs for the fourth straight year.

The second-seeded Trojans made sure, scoring five straight goals from the middle of the third quarter to the beginning of the fourth quarter to earn a 13-10 win Saturday in a Class A South semifinal. It was the ninth straight win for the Trojans (13-1), who will play top-ranked Scarborough for the regional championship on Wednesday. The Red Storm handed the Trojans their only loss of the season, 18-17 in overtime on May 2.

“For our seniors, we’ve got a couple boys (who have been) here for three years, (and) South Portland has ended their year every year,” Thornton Coach Ryan Hersey said. “We just said we can’t let our seniors go away with all four losses in playoffs against South Portland.”

The Red Riots (10-4) beat Thornton in the quarterfinals last year after victories in the semifinals the two previous seasons. But too many South Portland mistakes, dropped balls and the inability to contend with Thornton’s transition offense were too much this time.

“Its always a big battle between TA and SoPo in the playoffs,” South Portland Coach Tom Fiorini said. “It was a good game, a well-played game today. I am proud of my guys. … This year was their time. The ball rolled their way today. They played a good game today. My hat’s off to them.”

John Giroux led a third-quarter surge for Thornton immediately after a goal by South Portland’s Ross Myers brought the Red Riots to within 6-5 just 2:57 into the second half. Giroux scored from in close with 7:27 left in the quarter on a long cross-field pass from Thomas Levasseur, then got an unassisted goal 12 seconds later.

Noah Edborg, Giroux and Levasseur (four goals, one assist) scored to finish the 5-0 run and give Thornton its largest lead at 11-5.

David Fiorini converted for the Red Riots after deking a defender to end the surge.

“We were just going full speed and working through everyone on the team and getting it around (the offensive zone) and not stopping,” said Giroux, who finished with six goals and one assist.

A Fiorini goal started a run of four straight for the Red Riots, but Thornton answered when T.J. Pike (one goal, three assists) carried the ball from just inside midfield and passed to C.J. Labreck when the defense converged. Labreck’s shot beat South Portland goalie Quinn Watson (16 saves) for a 12-9 lead.

Less than two minutes later, with 2:25 remaining, Giroux scored again in transition.

“We won today with our transition offense,” Hersey said. “We out-transitioned them; we got probably 10 goals in transition, which was huge.”

Cooper Mehlhorn scored five goals for the Red Riots, but three of them were in the last half of the fourth quarter when his team trailed by four goals. Fiorini added two goals and three assists.

“We didn’t really focus on stopping (Mehlhorn),” Houde said. “We worked on bodying him up and keeping him in front (of the defense).”

Ean Patry made 11 saves for Thornton, many of them big ones early when the game was close.