BOSTON – Not even the three-time reigning state champions have been enough to slow down the Foxboro girls basketball team.

Within a Div. 3 state semifinal battle between title-defending powers Tuesday night at UMass Boston, the top-seeded Warriors (23-2) – who won last year’s Div. 2 state title – used a dominant defensive effort and a big scoring performance from its stars in the second half to dethrone No. 5 St. Mary’s of Lynn, 65-34.

Camryn Collins (20 points, four assists) and Kailey Sullivan (13 points, four assists) each only had three points in the first half, erupting in the third while the whole team suffocated the Spartans from recording a double-digit scorer.

Foxboro has now won all four of its state tournament games by at least 30 points and is back in the state final to face No. 2 Norwell at the Tsongas Center.

“It’s awesome, I’m blessed to be here,” Collins said. “Same as last year. It’s very rare for a team to come out here and get the same thing.”

It was very much a complete game for the Warriors, using stellar defense throughout while finding different ways to score in each half.

Junior center Addie Ruter was fundamental on both ends in the first, accruing nine of her 11 points and two of her three blocks as St. Mary’s struggled to answer her in the paint despite a gritty effort from center Reese Matela (nine points, 10 rebounds). The Spartans shot 0-for-6 from deep and couldn’t convert much on the inside, while double-teams on Ruter and smart passes from Erin Foley (seven points, seven assists) opened up jumpers for a 28-15 halftime lead.

“It was hard to defend us today,” said Foxboro head coach Lisa Downs. “To have a true center is just something that most teams don’t have, and if they have it, they’re not quite as talented as (Ruter) is. She’s an incredible part of our puzzle.”

The Warriors dialed up the tempo in the third quarter, which Collins and Sullivan took full advantage of.

Sullivan scored seven straight points for Foxboro early on, capped by the second of her three 3-pointers. That triple sparked a 15-2 run to lead 50-21, during which Collins used high energy in transition for 10 of those points. She finished the frame with 15 points, while the team forced six Spartans turnovers and outscored them 24-10.

“We saw that they were getting a little tired, so we had to take advantage of that and really jump on it,” Downs said. “We like to control the tempo of the game, and we like a fast game. … But our defense, again, just really came out tonight.”

“For us, the third quarter is the most important,” Collins added. “My teammates did a lot for me in transition. We worked a lot on defense and it turned into more points.”

Fellow starter Isabelle Chamberlain was active on defense and the glass for Foxboro (six rebounds, steal), as was Ava Hill (five points, six rebounds, steal) off the bench. Collins and Sullivan each had two steals, too, and everyone passed well in an assist-filled performance.

Bella Owumi had seven points and nine rebounds for St. Mary’s, and AJ Hyacinthe added six points with a pair of steals.

Foxboro’s Kailey Sullivan dribbles behind her back to evade St. Mary’s player Lily Norton during Tuesday’s state semifinal clash. (Chris Christo/Boston Herald)