In one short week, the U.S. men's national soccer team has gone from being an afterthought to a serious contender for the FIFA Confederations Cup championship.

Playing with increasing confidence and an unprecedented level of skill and energy, the Americans suddenly believe they can achieve what would be a milestone victory over Mexico in a semifinal Sunday at 100,000-seat Azteca Stadium.

The United States is 0-19-1 against Mexico in this country, and Coach Bruce Arena was coy today, saying: "I've felt all along that Mexico is one of the top 10 teams in the world. . . . We will have our hands full. Mexico was the pre-tournament favorite and will have an advantage playing at home, but we will do our best to prepare and try to win."

His players aren't so shy--especially after advancing to the semifinals by defeating Germany, 2-0, in the teams' final first-round game Friday night in Guadalajara. It was the United States' second shutout victory over the three-time World Cup champions in the past seven months, and this one was accomplished with a lineup largely composed of players who don't usually start for the national team. No one is bragging or making predictions, but the team is loose and ready.

"It's amazing," said D.C. United midfielder Ben Olsen, who scored the Americans' first goal Friday night. "If [Arena] has faith in you, then you have faith in yourself, and you know you can do it."

And right now, oddly enough, the U.S. team may have something of a mental edge over the Mexicans. Although Mexico won its first-round group with a 2-0-1 record, many fans have expressed dissatisfaction. And crowds at Azteca Stadium have been known to turn on the home team when it doesn't meet expectations.

As it is, there have been some ugly confrontations between Mexican players and members of the Mexican media, notably those from TV Azteca, who have been accused of fomenting dissent between the players and fans.

There also are the usual questions of psychology and fitness: Forward Luis Hernandez is questionable for Sunday's game--he is listed as injured--yet some observers here say he is just planning to rest during the first half and enter during the second.

On the other hand, Mexico also is one of only two teams to beat the United States during Arena's brief tenure (Brazil is the other--a 1-0 victor in a closely contested match Wednesday night in Guadalajara). And then there are the matters of history and environment.

The impact of Mexico City's altitude, smog and summer heat will be compounded by Sunday's game time--noon locally. In addition, the closest an American team has come to victory here was a memorable 0-0 tie in a World Cup qualifying match in November 1997. The Americans had to play the final 60 minutes with 10 players because D.C. United defender Jeff Agoos was ejected.

"I thought it was a lousy [red] card then, so it doesn't really bother me now," Agoos said. "But it's not a question of vindication. I just want to go out and play a full game."

If Agoos does so, it will be his fourth game in eight days, a demand Arena might not put on him. Agoos was forced to play Friday night when Gregg Berhalter became ill, and he is the only American with 270 minutes played here.

He likely will be joined in the starting lineup by the full complement of players who didn't start Friday night against Germany: midfielder John Harkes, goalkeeper Kasey Keller, defender Robin Fraser, forward Jovan Kirovski, midfielder Ernie Stewart and forward Brian McBride. It possibly would be America's best starting 11 of this tournament, but Arena has managed to find depth here where many previous coaches saw only debris.

"There is no A, B, C team--none of that stuff," defender-midfielder Frankie Hejduk said. "Whoever steps on the field is our starting team, and they're the ones to get the job done."

If the Americans continue to produce the unexpected, they will play Brazil or Saudi Arabia in the final Wednesday night at Azteca. A loss would send them back to Guadalajara for a third-place game Tuesday night.

Confederations Cup

* Today's Semifinals:

United States vs. Mexico in Mexico City, 1 p.m.

Brazil vs. Saudi Arabia in Guadalajara, Mexico, 4 p.m.

* TV: Pay-per-view*

* Consult your local cable operator.