D.C. United has always held the U.S. Open Cup in high regard. This year, amid a sour league campaign, United is invested in the 100th annual tournament like never before.

Winning the title this fall would not erase the rotten MLS regular season. But it would help. A lot.

“We look at the Open Cup as an opportunity to get some happiness back into our lives,” said goalkeeper Joe Willis, who will start Wednesday’s semifinal against the Chicago Fire at Toyota Park in Bridgeview, Ill.

At stake is a championship pairing on the road with the Portland Timbers on Sept. 24 or Real Salt Lake on Oct. 1. Those sides will also clash Wednesday, at Rio Tinto Stadium in Sandy, Utah.

Since escaping the round of 32 in a penalty-kick tiebreaker at third-division Richmond, United has posted a pair of 3-1 victories over MLS opponents. D.C. is a league-worst 3-15-4 in the regular season but gained a teaspoon of momentum by defeating Montreal, 3-1, on Saturday night at RFK Stadium.

“The belief is coming along,” Coach Ben Olsen said.

Last in the league in scoring, United attacked effectively against the Impact. Luis Silva (three goals in three league games since being acquired from Toronto) and Dwayne De Rosario forged a promising partnership.

“He is a real player,” De Rosario said. “He understands the game. He plays with a nice rhythm and similar to the way I play. If we can synchronize that rhythm, which we did at times during that [Montreal] game, it’s a positive outcome. The more we play together, the more that synergy will grow.”

Chris Pontius might join them on the field after missing three matches with a hamstring injury. He was included on the travel roster to Chicago. Asked if the dapper midfielder is ready to play, Olsen said: “I don’t travel people just for their good looks.”

United did not look good at all in a visit to Chicago 2 1/2 weeks ago — an early two-goal deficit and a 4-1 defeat.

“We got bullied around,” Willis said. “We are using it as motivation. Last time we were there, they kicked our ass. I am still pissed about it. I know 99 to 100 percent of the locker room is still pissed about it.”

United may carry one slight advantage into this match: new players and an unpredictable lineup.

“Nobody knows who we are right now,” Olsen said. “We are a team that is tough to prepare for in that they don’t know who is going to be on the field.”

Chances are, though, Olsen will not change much of the lineup that clicked against the Impact: Kyle Porter and Nick DeLeon on the flanks, Perry Kitchen and Jared Jeffrey in central midfield and Dejan Jakovic and Daniel Woolard in central defense. Willis will start his fourth consecutive Open Cup game: He blanked Richmond through 120 minutes and was the difference in the tiebreaker, then conceded two goals in the subsequent two matches.

Pontius and Conor Doyle (goal and assist last weekend) will likely be second-half options.

Since smashing United last month, the Fire (8-9-4 in league play) earned a draw at Houston and won at Philadelphia.

“We both take the tournament very seriously,” Chicago forward Chris Rolfe said on the Fire’s website. “It’s a big opportunity for them to have a successful season. I think they are going to come full throttle at us.”

No TV coverage is planned, but both the Chicago and DCU websites will carry a live stream, starting at 8:30 p.m. ET.