Needing one win to advance to the next round of the Hockey East playoffs, one obstacle stood in the way of the No. 3 Boston University men’s hockey team and trip to the semifinals — Rasmus Tirronen.

Much like he did in Friday night’s game, the Merrimack College netminder frustrated the Terriers in the first period Saturday, this time making 18 saves. Shot after shot, Tirronen was in position, keeping the Warriors in the game.

Despite not scoring in the first 20 minutes, the Terriers felt like their sound attacking strategy would produce results and prevail. And just like déjà vu, it did.

Back to TD Garden they go.

The Terriers (23-7-5) scored three times in the second period and twice more in the third, finishing with a 5-0 win over Merrimack (16-18-4). Senior assistant captain Cason Hohmann had two of the goals, while freshman center Jack Eichel added three assists to his season total.

After failing to win a postseason game a season ago, BU is headed to the conference semifinal for the first time since 2012-13. The sweep is David Quinn’s first-ever postseason series victory as head coach.

“Just really happy for our guys,” Quinn said. “Really proud of the way we played the whole weekend, all six periods. It’s a very difficult team to play against. I felt we really kept our composure and stuck to what we needed to do to have success.”

By series end, the Terriers outscored the Warriors by an 11-2 margin. According to junior Matt Grzelcyk, success came from physical and mental aspects of the game.

“I think obviously the stakes are higher and it’s kind of a new season,” Grzelcyk said. “We wanted to make sure that we’re mentally in it. At the beginning of the year, we’ve had tough first starts, but I was very pleased with both games. I think we came out hard and really set the tone for the rest of the game.”

BU did come out strong, and started with two early chances on the power play, including an extended 5-on-3 opportunity.  Tirronen, though, wouldn’t have any of it, as he stood on his head for the entire first period. His good positioning and quick reflexes stopped a number of Terrier chances in high-percentage areas.

The highlight for Tirronen came toward the end of the period when senior assistant captain Evan Rodrigues circled the slot and patiently waited for an opening. Rodrigues had a a great look, but Tirronen made a spectacular diving stick save to keep the Terriers out of the net.

For their part, the Warriors had two power plays of their own, both coming from penalties on freshman forward Nikolas Olsson. BU killed of both minor penalties, and junior netminder Matt O’Connor stopped all nine shots that came his way in the period.

Although he was stalwart in the first, Tirronen couldn’t do it all in the second. BU outshot the Warriors 24-2 in the period and the cracks turned to holes.

Hohmann got the scoring started at 4:11 in the period with a shorthanded goal off of a 2-on-1 with Eichel. The freshman toe dragged to avoid a diving poke check, fed it to Hohmann, who did the rest for his seventh goal of the year.

A little over a minute later while on the power play, senior assistant captain Evan Rodrigues rocketed a shot from the point that  by junior winger Ahti Oksanen redirected past Tirronen for BU’s second goal.

Junior forward Danny O’Regan finished the trio of second-period goals, one-timing a low shot from the right circle that just squeaked by Tirronen at the 17:40 mark. Eichel again recorded an assist, his 60th point of the year. It’s the first time a BU skater has reached that plateau since Chris Drury recorded 62 points during the 1996-97 season.

The Terriers played conservatively for the final 20 minutes, but they still controlled play in the offensive zone. As such, O’Connor didn’t have much to do in his end. He did make one flashy glove save with 8:22 left in the game, robbing senior Clayton Jardine’s backhander down low.

In desperation mode, the Warriors pulled Tirronen with over four minutes left in the game. Seconds later, junior captain Matt Grzelcyk dumped the puck from the defensive zone and into the gaping net. Eichel was credited with his third assist of the game.

A four-goal lead in hand was enough for Quinn to pull O’Connor for the final minute in favor of graduate student goaltender Anthony Moccia. He made one save in his second appearance this season.

What would’ve went down as O’Connor’s second-career shutout will go down as just a memorable postseason performance.

“At the end of the day, Moccia’s going to remember that for the rest of his life,” Quinn said. “OC I’m sure would’ve loved to have the shutout, but that moment’s going to mean a lot to Moccia than O’Connor finishing the last minute and a half of that game. I talked to OC about it, I’m sure initially he was a little bit frustrated, but by the end of it, by the time we were in the locker room, he was beaming and happy as heck for Moccia.”

Hohmann put the finishing touches on the win with BU’s second empty-net goal of the game with 24 seconds left.

From here on out for BU, every game has increased importance. One loss and a tournament or national title run could be over. It’ll be prudent for the Terriers to remain composed and focused.

“Now it’s one-and-done for the rest of this tournament and hopefully in the national tournament,” Quinn said. “It’s going to be one-and-done, so you’re not going to have a chance to have a bad night and recover from it. In the regular season, you have a bad night and come back and bounce back the next night and salvage a weekend.

“You can’t do that here on in.”