United defender Chris Korb, right, holds off Fire midfielder Joel Lindpere during the second half. (Nam Y. Huh/Associated Press)

The lowliest team in MLS this season will play for a trophy this fall.

Dwayne De Rosario scored late in the first half and Nick DeLeon struck early in the second Wednesday as D.C. United upset the Chicago Fire, 2-0, in a U.S. Open Cup semifinal at Toyota Park in Bridgeview, Ill.

United (3-15-4 in league play) will play at Real Salt Lake on Oct. 1 in the final of the 100th annual tournament, which brings together teams from all levels of American soccer for a knockout competition. RSL defeated the Portland Timbers, 2-1.

The Open Cup winner will qualify for next year’s CONCACAF Champions League, an international tournament featuring Mexican and Central American titans.

A two-time champion, United last reached the final in 2009, losing to Seattle.

Chicago, which is in a playoff battle in MLS’s Eastern Conference, was aiming for a seventh Open Cup championship appearance in the club’s 16-season history (an MLS-record four crowns and two runner-up finishes). The Fire had never before lost an Open Cup game at home.

United has scored just 13 goals in 22 regular season matches but eight in the past three Open Cup outings, all against MLS teams.

With halftime nearing and neither side generating serious threats, DeLeon’s deflected shot from beyond the penalty area floated toward the six-yard box. Sean Johnson, the U.S. national team’s second-choice goalkeeper at the CONCACAF Gold Cup last month, had a routine play in front of him.

Instead of catching the ball above his head, Johnson attempted to pull it into his body. With Luis Silva pressuring him, Johnson let the ball slip between his arms. Before he could retrieve it, De Rosario tapped the ball from him and fired an angled shot past defender Austin Berry for his fifth goal of the tournament.

Less than three minutes after intermission, DeLeon collected possession at the box, and with four defenders boxing him in, lashed a 15-yard shot into the lower left corner.

Silva was forced to leave in the 57th minute with a possible concussion, suffered while challenging Bakary Soumare for the ball. His replacement, Conor Doyle, tested Johnson in the 60th with a running one-timer, set up by DeLeon’s pass.

Chris Pontius, sidelined for two weeks with a hamstring injury, replaced DeLeon in the 74th minute.

Joe Willis, who has manned the net throughout the tournament, had three harrowing moments in the second half but stopped one shot and turned to midfielder Perry Kitchen and defender James Riley for goal-line clearances.

The outcome avenged a pair of regular season losses at Toyota Park, most recently a 4-1 outcome less than three weeks ago.

United will resume league play Saturday night at Philadelphia.

SPIRIT 1, RED STARS 0: At Maryland SoccerPlex, Teresa Worbis’s goal in the 58th minute helped Washington (2-14-4) snap a 13-game winless streak. The match was called off with 10 minutes left after two lightning delays totaling more than two hours.