A three-goal outburst early in the second half propelled the Union to a 4-0 win over Saprissa on Wednesday night at Subaru Park in the second leg of their Concacaf Champions League round of 16 series, and sent the Union on to the quarterfinals with a 5-0 aggregate victory.

Carrying a 1-0 lead from the first game, the Union started brightly, aided by a boisterous crowd of around 5,000 fans that was on its feet from the kickoff. The fans booed every touch by Saprissa’s Ricardo Blanco, whose vicious tackle late in the first game sent Kai Wagner airborne. And they came close to erupting in a good way in the sixth minute when Jakob Glesnes received a corner kick and hammered a shot off the left post.

It was still scoreless when referee Fernando Guerrero pulled out his first card of the game for Saprissa’s Jedwin Lester in the 35th minute. Lester aimed his right foot into Jamiro Monteiro’s right ankle with a hard slide, and both teams started to gather around Guerrero. But this time it didn’t escalate, with José Andrés Martínez among the leading peacemakers.

» READ MORE: 3 Saprissa players who faced the Union tested positive for COVID-19 before their Champions League rematch at Subaru Park

At halftime, the Union had a 5-2 advantage in shots but no more goals on the scoreboard. Though carrying the aggregate lead, they knew they needed breathing room — and they got it fast. Fortunately, it didn’t take long to fix the matter.

Just 19 seconds into the half, Anthony Fontana was tripped on the line of the 18-yard box by Michael Barrantes, and the Union had a penalty kick. There wasn’t much contact, but there was just enough and Fontana made the most of it. Monteiro stepped up and smashed the ball past Aaron Cruz.

From there, the floodgates opened. Kacper Przybylko thumped home a header off a Monteiro corner in the 51st, and Fontana broke behind Saprissa’s defense in the 54th to slot home another off a Monteiro assist.

“We knew once we scored the first goal, a lot of them were going to quit, and that’s what you saw,” Fontana said.

Now the focus turned from winning the series to counting yellow cards. Monteiro, Martínez, and Glesnes each had one in the tournament at that point, and a second for any over the course of the series would draw a suspension for the first quarterfinal game.

Martínez exited in the 67th for Stuart Findlay, who made his Union debut. Leon Flach went off at the same time for academy-bred rookie Jack McGlynn, who earned his first minutes with the first team. Nine minutes later, Przybylko and Alejandro Bedoya wrapped up their nights, replaced by Sergio Santos and Quinn Sullivan.

Glesnes finally was withdrawn in the 89th for Aurélien Collin. Seconds later, Monteiro capped off the scoring with his second goal of the night: a sprint down the middle of the field and classy finish. He totaled two goals, two assists, four shots, four chances created and 43-of-48 passing in the game.

I’m really proud of the team’s performance over both legs,” Curtin said. “Full credit to the players for their commitment in this competition. ... I was happy with how our players showed real professionalism and took care of business, the games within the game and being smart that way.”

The Union will face Atlanta United in the quarterfinals, a rare all-MLS matchup in the tournament’s late stages. Atlanta ousted the other Costa Rican team in the round of 16, Alajuelense, 2-0 on aggregate. Atlanta will host first between April 27-29, and the Union will host the second leg between May 4-6.

Before then, the Union kick off the regular season at Columbus on Sunday (5:30 p.m., FS1 and Fox Deportes) in a matchup of last year’s Supporters’ Shield and MLS Cup winners. The regular-season home opener is April 24 against Inter Miami (8 p.m., PHL17).

» READ MORE: Alianza de Futbol is bringing its youth soccer showcase to the Philadelphia area for the first time