By Seth Lakso, Globe correspondent

Acton-Boxborough forward Elizabeth Belanger stepped to the free-throw line midway through the third quarter and, without hesitation, notched her 1,000th career point, at the same time giving the Colonials a 15-point edge over Billerica in the first round of the girls' Division 1 North state tournament.

Belanger, who’s still a junior, finished the game with team-highs in points (28) and rebounds (12), as the Colonials rolled over Billerica at home, 71-58.

The Colonials weren’t sure what to expect from their leading scorer entering the game, as she had been battling an illness much of the week.

“We really didn’t even know if we’d get all that much from her today, but she showed up and was excellent,” said Acton-Boxborough assistant coach Tim Bassett. “She’s been our catalyst all year [and] can really do it all. She spends a lot of her time down below the hoop, helping us rebound, but she can take it out and shoot with the best of them.”

Acton-Boxborough led, 37-28, at halftime, but saw that margin cut to just 8 as the third quarter wound down. Bassett knew not to panic, as he let the girls play through the inevitable lull that followed Belanger’s milestone achievement.

“I just let them play through it,” Bassett said. “We have some really good senior leadership on this team. I knew we would come out of it, so I just went with it and we started to play our game again.”

Acton-Boxborough captain Hannah Foley was one of those providing the senior leadership, as she helped the team re-locate its focus in the final frame, as the point guard kept the offense flowing. Foley tallied 20 points for the game.

With the victory, Acton-Boxborough advances to the quarterfinals against Lowell on Saturday at home. If the Colonials win, a rematch with Lincoln-Sudbury, one of the two teams to defeat A-B (18-3) this season (the other was Newton South, twice), could await in the semifinals.