BRUNSWICK — With a reflex reaction, Katie Riley ensured the Bowdoin field hockey team’s historic run through the NCAA Division III tournament rolled on Sunday.

Riley’s second-half goal helped lift the Polar Bears to a 3-0 win over MIT in the third round of the tournament. Bowdoin (19-0) will play in the NCAA Division III semifinals Saturday against Middlebury at Nichols College in Dudley, Mass.

The Polar Bears have won a record 14 straight NCAA tournament games. But a win over the Engineers (19-4) was in doubt before Riley’s goal.

With Bowdoin leading 1-0, Elizabeth Clegg fired a shot at MIT goalie Jenna Klein. The ball rebounded to Riley, who snapped it into the back of the goal for a 2-0 lead with 16:50 to play in the game.

“She was giving up a lot of rebounds, so that’s what we were looking for,” Riley said.

Riley, who entered the game during the second half, scored just her fifth goal of the season. And that’s exactly what makes Bowdoin such a tough opponent, MIT Coach Cheryl Silva said. Bowdoin has seven players with at least eight points.

Which one do you stop?

“This isn’t a situation where we can take our best kid and assign her to their best kid,” Silva said. “Everybody had to play well for us.”

For a while, everybody from MIT did play well.

The Engineers had several dangerous opportunities swept away in front of the Bowdoin net in the first half. Bowdoin’s Cathleen Smith scored with 9:33 to play in the first half but the Engineers kept hanging around.

It wasn’t until Riley’s goal midway through the second half that Bowdoin Coach Nicky Pearson could breathe a little easier.

“The game was very back and forth,” Pearson said. “They’re a very good offensive team. It never feels like one (goal) is enough.”

Two, however, were plenty. Five minutes after Riley’s goal, MIT’s defense broke down and Clegg score the game’s final goal.

While it was dangerous to let MIT hang around, there was no chance the Engineers were going to score multiple goals. They earned just one penalty corner in the second half, with 2:20 remaining. Bowdoin had 15 shots on goal to MIT’s four.

The Polar Bears’ dynamic offense will face a familiar defense in the Final Four on Saturday.

This will be the third time Bowdoin has played Middlebury this season. The Polar Bears beat them twice at home — 3-2 in overtime on Sept. 24 and 2-1 on Nov. 6 in the NESCAC tournament championship game.

Yes, Bowdoin won both, but Middlebury had a shots-on-goal advantage last week. The Polar Bears know exactly what to expect, but so do the Panthers.

The Polar Bears need to beat one of the few teams that’s given them trouble this year if they’re going to win their second straight NCAA title and fourth in five years.

“It’s a double-edged sword, isn’t it?” Pearson said.

“We know we’re getting a very talented team. It’s sort of a mixed feeling. But we will go in knowing we’ve beaten this team twice. We know exactly who they are.”