United, with the worst record in Major League Soccer, scored its fourth win of the tournament. It has three victories in MLS competition this season.
De Rosario scored in the 44th minute, collecting a rebound that Fire goalkeeper Sean Johnson played directly forward. He tapped the ball to the left when Johnson charged out, and scored into an open net from 4 yards.
DeLeon broke the game open in the 48th minute, sneaking between a pair of defenders after taking James Riley's pass, and right-footing a low shot into the corner of the net to Johnson's right.
United goalkeeper Joe Willis made six saves, including one on Alex in the 76th minute. Willis watched Austin Berry's deflected shot carom off the goal post a minute later. Earlier, Willis solved Chris Rolfe and Alex in the first half.
Neither Chicago nor D.C. United is above .500 in MLS competition, but advanced to the U.S. Open Cup semifinals by beating a combination of MLS and lower-level professional squads. United had beaten Richmond of the USL Pro circuit, and MLS foes Philadelphia and New England, to advance to the semis.