D.C. United's dreams of a berth in the world club championships came to a crushing end tonight with a 3-1 loss to Mexican power Necaxa in the semifinals of the CONCACAF Champions Cup.

United, the tournament's defending champion, took the lead on defender Carey Talley's goal midway through the first half, but Necaxa took advantage of goalkeeper Tom Presthus's shaky play and scored three times in a 28-minute span.

In Sunday's final at Sam Boyd Stadium, Necaxa will play Alajuela of Costa Rica, which eliminated the Chicago Fire this evening in a penalty kick tiebreaker following a 1-1 tie. United and the Fire will meet in the third-place game on Sunday before returning home for the final week of Major League Soccer's regular season.

The tournament champion advances to the inaugural world championship early next year in Brazil, where it will join Manchester United and Real Madrid, among others.

"We still felt we were in control of the game [at halftime], United Coach Thomas Rongen said. "But we know at this level. . . . mistakes will be fatal and we'll be punished for it, and that's what happened."

Rongen made one change in the starting lineup tonight, the same move he executed with successful results in the second half of the quarterfinal victory over Olimpia (Honduras) on Wednesday. Talley, who assisted on the only goal against Olimpia, came in at right back, Diego Sonora moved into the midfield and John Maessner sat down.

Rongen then had to make an unplanned change in the 19th minute tonight when leading scorer Roy Lassiter strained his left calf on a midfield confrontation and was replaced by A.J. Wood. United lost Lassiter's speed and prowess, but gained Wood's size, strength and heading ability.

Necaxa, an entertaining team charged by Ecuadorans Alex Aguinaga and Agustin Delgado, controlled the first 20 minutes with crisp ball movement on the narrow field and a swarming defense that suffocated United playmaker Marco Etcheverry.

Unable to move the ball in the run of play, United turned a dead ball situation into a 1-0 lead in the 26th minute. Etcheverry's corner kick sailed across the penalty area to Ben Olsen, who placed a header into the middle of the box for Talley to volley in from eight yards.

With a surprising lead, United's confidence seemed to grow and Etcheverry discovered more space to operate. But Necaxa got an improbable equalizer in the 40th minute on a ghastly mistake by Presthus.

Aguinaga aimed a corner kick along the end line for a teammate to flick into the middle. Presthus had the angle covered at the near post, but when the ball came at him untouched, he was unprepared and bobbled it into the air. In an instant, Necaxa defender Sergio Almaguer nodded the ball back past the flailing Presthus.

Necaxa dominated at the start of the second half, moving ahead in the 57th minute when German Villa found Delgado with a long ball from right to left. Delgado's wicked 12-yard shot from a troublesome angle streaked by a leaping Presthus and settled into the far corner.

United threatened in the 66th minute, but goalkeeper Hugo Pineda made a spectacular reflex save on Wood's hard header. Two minutes later Presthus misjudged Aguinaga's corner kick, letting the ball sail over his long reach and allow Edgar Olivia to head it into an open net.

In the other semifinal, Alajuela converted all five of its penalty kicks after the teams played to a 1-1 tie. The difference was a missed attempt by Chicago veteran Lubos Kubik, who sent his kick in the second round soaring over the crossbar. Alajuela clinched it in the final round on Javier Delgado's shot past goalkeeper Zach Thornton and into the lower left corner.

During regulation, Alajuela took the lead in the 22nd minute on Wilson Munoz's low, 16-yarder that slipped into the left corner. The Fire tied it in the 38th minute on Roman Kosecki's 22-yard free kick.

Chicago nearly went ahead with about three minutes left in the game, but Ante Razov's 32-yard free kick directed toward the upper left corner was tipped over the crossbar by goalkeeper Alvaro Mesen.

In the first half, Razov hit the crossbar from long distance and Alajuela's Edson Valente nicked the right post on a counterattack.

Soccer Notes: Former D.C. forward Raul Diaz Arce was called up to the Salvadoran national team and will not accompany the Tampa Bay Mutiny to Washington for Wednesday's game against United at RFK Stadium. Diaz Arce, who still has a large following in Washington, will miss the Mutiny's final two regular season games before returning to the club in time for the playoffs.