A quartet of unexpected semifinalists are still alive at the MLS is Back tournament.

The Philadelphia Union advanced to the semifinals thanks to a strong defensive performance throughout the competition and its largest offensive output in a single game in the quarterfinals. The Union are set to face the Portland Timbers, who are the most established of the four franchises left, in the first semifinal.

Portland won the 2015 MLS Cup and it was the runner-up in 2018. To get to the final four, the Timbers had to top a group with pre-tournament favorite Los Angeles FC and fend off the challenge of 2019 Eastern Conference regular-season champion New York City FC in the quarterfinals.

Orlando City and Minnesota United knocked out two of the best teams in the competition, LAFC and the San Jose Earthquakes, respectively, to set up Thursday's clash with a berth in the final on the line.

Quarterfinals Results

Philadelphia Union 3, Sporting KC 1

Orlando City 1, Los Angeles FC 1 (Orlando City wins 5-4 on penalties)

Minnesota United 4, San Jose Earthquakes 1

Portland Timbers 3, New York City FC 1

Semifinal Previews

Philadelphia Union vs. Portland Timbers (Wednesday, 8 p.m. ET, FS1)

Philadelphia's calling card in the tournament has been its defense.

USMNT prospect Mark McKenzie has anchored a back four that has not allowed multiple goals in any of the five games in Orlando. Goalkeeper Andre Blake is in contention to be the Most Valuable Player at MLS is Back, as he has totaled 27 saves. But the player directly in front of the back four and Blake could be most critical to Philadelphia's success Wednesday night.

Jose Martinez has been a revelation in the competition with his play in defensive midfield, but he has struggled at times to harness his aggressiveness, which could lead to dangerous set-piece opportunities for Diego Valeri and the Portland attack.

Valeri came off the bench Saturday to score the game-winning goal against New York City FC, and if he starts Wednesday, he could trouble Martinez. Valeri has been the catalyst of the Portland attack for most of his time with the club, and he is surrounded by a pair of clinical forwards in Jeremy Ebobisse and Jaroslaw Niezgoda.

Portland's main midfield destroyer, Diego Chara, could be its key to a berth in the final, as he will get the task of shutting down 19-year-old midfield phenom Brenden Aaronson, who has been one of the best young players at MLS is Back. He has three assists in five games and has impressed with his technical ability in the final third.

If Chara takes Aaronson out of the game, it could create a hole in the Union attack that limits chances in the final third for Kacper Przybylko and Sergio Santos.

Orlando City vs. Minnesota United (Thursday, 8 p.m. ET, ESPN2)

The second semifinal reunites Minnesota United manager Adrian Heath with former side Orlando City at a venue where he enjoyed success with the Lions.

The Heath-coached Lions spent the final year of their pre-MLS existence playing home games at ESPN's Wide World of Sports.

Heath has embraced the underdog role throughout the knockout phase, and he has gotten results out of his squad. The Loons knocked off Columbus Crew, who were the only team to have a perfect group stage, in the round of 16 before cruising past San Jose on Saturday night.

Heath noted after Saturday's win that he will have Minnesota United ready to play aggressive Thursday night, per Julia Poe of the Orlando Sentinel:

"We know we've got a really difficult task. I think Oscar [Pareja]'s had a great start with Orlando. They've got some good players. They've obviously got a real togetherness, you can see that. It's gonna be tough. But I also know that Oscar is wise enough and experienced enough to know that we ain't coming in here and playing on the back foot. We're going to be ready for the game."

Minnesota's key to advancing could be the play of its central midfielders. Osvaldo Alonso carries plenty of knockout-round experience from his time with the Seattle Sounders, and Jan Gregus has turned into one of the most reliable central midfielders in MLS.

Alonso and Gregus could bottle up Nani and keep the Portuguese playmaker off the ball in the final third. Nani was on target with three of his seven shots in Friday's win over Los Angeles FC.

Orlando's X-factor could be Ruan, who has been a vital piece of its attack from right-back. The Brazilian drew a penalty Friday and could open gaps in Minnesota's defensive line by overloading the right flank.

If Ruan is allowed to surge forward and provide support for Nani, Chris Mueller and others, the Lions could find a good amount of success in front of goal.

Follow Joe on Twitter, @JTansey90. Statistics obtained from MLSSoccer.com.