In the closing minutes of D.C. United's Champions' Cup quarterfinal opener against Olimpia last week in Tegucigalpa, Honduras, defender Facundo Erpen scored a goal that had little relevance on the one-sided match but enormous ramifications on the series.

The 10-yard header increased United's lead to 4-1, but of greater importance to the two-game, total-goals playoff, it essentially secured the club's place in the semifinals of the regional club championship.

If United had carried a one- or two-goal lead into tonight's finale at RFK Stadium, the series outcome may very well have been in doubt. But with a three-goal advantage at home in clammy conditions against a warm-weather opponent with a rickety defense, D.C. is in prime position to advance.

Then again . . .

"There is that concern that we will be passive and too defensive," midfielder Ben Olsen said yesterday after a light workout. "It's always a recipe for disaster if you sit and defend for 90 minutes. Soccer is not built for that. We've got to play like nothing happened last week."

It will be difficult for United -- and for Olimpia, for that matter -- to forget about the first match, a startling result considering the MLS club was just three weeks into preseason training and had a notorious record in games played in Latin America.

With a three-goal margin, the dynamic of the series will change dramatically this evening "because they've got to score a lot of goals," United defender Bobby Boswell said. "It's key for us to not let them get anything early and, even though we don't need one, if we can score a goal, I think it would really break their spirits."

According to Coach Tom Soehn and the players, it is simply a matter of avoiding complacency, sticking to the game plan and, most importantly, ignoring that all but unshakable series lead.

United expects Olimpia to attack in greater numbers, which, if handled properly, will present D.C. with tempting counterattack opportunities and a chance to turn the series into a runaway.

Soehn said he is not worried about the big lead affecting his club's approach.

"We're going into this game with the same mentality as the last game," he said. "We have an experienced team. They've been here before, they understand the importance and consequence of each game. These guys are professionals."

Besides maintaining its defensive composure, United will hope to replicate its efficient scoring touch. Argentine playmaker Christian Gomez struck from 25 and 20 yards during the first leg, and Luciano Emilio, the club's newly acquired Brazilian forward, was influential beyond his tiebreaking goal just before halftime.

The former Olimpia player provided a much-needed physical presence on the front line, made smart runs without the ball and joined Gomez and Bolivian forward Jaime Moreno to form an imposing South American assault.

Despite its advantages on the field and in the series, United is not celebrating just yet.

"We had some chances, they had some chances, but the difference [in the first meeting] was we were just better at finishing them," Olsen said. "Olimpia's capable of the same thing. If they come in and they're flying and they're scoring good goals, it could be a long day for us."

United Notes:If Olimpia wins by three goals tonight, the series will be decided by 30 minutes of overtime and, if necessary, penalty kicks. . . . If United advances, it will host the first leg of its semifinal series March 15 at 8 p.m. and play on the road three weeks later.