There were several reasons for D.C. United's 3-1 semifinal loss to Necaxa of Mexico late Friday night in the CONCACAF Champions Cup, but none was as glaring as goalkeeper Tom Presthus's misadventures.

"It obviously wasn't the best game I've played and it was an awful time for it to happen," Presthus said this morning. "I could have done better -- much better."

Presthus's mistakes resulted in Necaxa's first and third goals and left United in Sunday's third-place match against the Chicago Fire at Sam Boyd Stadium. Later Sunday, Necaxa will play Alajuela of Costa Rica -- which beat Chicago in a penalty kick tiebreaker Friday -- for the North American title and a berth in the inaugural world club championships early next year in Brazil.

United Coach Thomas Rongen said he would start several reserves against the Fire, but hadn't made any decisions. Leading scorer Roy Lassiter, who injured his calf in the first half Friday, will not play and probably will miss United's Major League Soccer game against Tampa Bay Wednesday at RFK Stadium.

Presthus said he would like to play in the consolation game to continue preparing for the MLS playoffs, which begin Oct. 16. He certainly wants the opportunity to redeem himself for Friday's nightmare.

With United leading 1-0 on Carey Talley's volley in the 26th minute, Presthus made his first critical error late in the first half. A corner kick that had been flicked along the end line caught Presthus by surprise and bounced off his hands, and Sergio Almaguer easily nodded the ball past him.

Early in the second half, Presthus had little chance of stopping Agustin Delgado's hard shot that gave Necaxa a 2-1 lead, but 11 minutes later he misjudged a corner kick and let the ball soar past him for Edgar Oliva to head into an open net.

"I don't want to dwell on it, but I do want to think about it so I know what I did wrong and how I can improve on it," said Presthus, an MLS all-star who in the quarterfinals here made a spectacular last-second save on a penalty kick to secure a 1-0 victory over Olimpia of Honduras. "I want to learn from this. I'm not going to mope around and lose the confidence of my teammates."

The other key factor in United's loss was a continued inability to adapt to the narrow field. Without the space to which he is accustomed, playmaker Marco Etcheverry didn't distribute the ball with his usual grace and precision.

Nonetheless, United didn't fade away. Jaime Moreno's shot with a few minutes remaining hit the inside of the right post and bounced along the goal line, but not across it.

Said Necaxa Coach Raul Arias: "They are obviously one of the strongest teams in the confederation. We had difficulty with their height, we had difficulty with their strength, but we were able to impose our game, which depends on good touch and good passing."

Friday's semifinals were a missed opportunity for MLS, a four-year-old league that has made strides on the field, thanks primarily to United's excellence, but still yearns for international acceptance. A berth in the world championships would have meant games against fabled clubs such as Manchester United and Real Madrid before a global television audience, and a guaranteed $1.5 million.

"We were very, very aware of what this meant to us and what it would mean to MLS in the short and long term," Rongen said. "We're disappointed because we lost the game, not because we had to carry the burden of trying to showcase internationally what MLS is all about. We're disappointed that we didn't win this tournament. . . .

"We said as a group that we want to get back here and prove we are the best team in CONCACAF. But the only way we get back here [next year] is if we win MLS Cup [this year], and that is now our main focus."

CHAMPIONS CUP

Third-Place Game

D.C. United vs. Chicago Fire

Where: Sam Boyd Stadium, Las Vegas.

When: 5 p.m.

TV: HTS.

Championship Game

Necaxa (Mexico) vs. Alajuela

(Costa Rica)

Where: Sam Boyd Stadium, Las Vegas.

When: 7 p.m.

TV: Fox Sports World.