HOUSTON — For two days, while awaiting an opponent in the Copa America semifinals, the U.S. national team went about its business as if the foe didn’t matter. The Americans were fixed on fitness, on resettlement in their ninth city over four weeks and on attempting to get two suspensions rescinded.

As the outcome of the Argentina-Venezuela match came clear Saturday night, they began to pivot.

The world’s best soccer player, Lionel Messi, and the world’s top-ranked soccer team, Argentina, would be joining them in Houston for Tuesday night’s showdown.

Argentina secured passage with a 4-1 victory in Foxborough, Massachusetts, setting up the biggest match for the Americans since the 2014 World Cup and the most glamorous duel with a player since Cristiano Ronaldo two years ago in Brazil.

“We are not scared of them at all,” Coach Jurgen Klinsmann said Sunday. “We admire their players. This is now a special moment. I told the players yesterday before we started training, ‘This is a once-in-a-lifetime opportunity now.’ You got into the semifinal, you made yourself proud, now go for more.”

The U.S. delegation watched the Argentina match at the team hotel, in small groups and individually. Since defeating Ecuador on Thursday in Seattle, the Americans had a safe hunch who they would face next.

In his first start of the tournament, Messi scored once and assisted twice, and Gonzalo Higuain pocketed two early goals as Argentina remained perfect through four matches.

Trailing 2-0 late in the first half, Venezuela squandered a chance to make a game of it by failing to convert a penalty kick.

“There’s a stretch of the beginning of the game where Argentina came out very sharp and into it,” U.S. captain Michael Bradley said. “They get ahead, but at the same time Venezuela had big chances to get back into the game. On another day, the whole thing can play out a little differently.”

While acknowledging the enormous challenge that lies ahead, Bradley also said, “We don’t want to make this out to be a ‘Mission Impossible.’ ”

The Americans have faced FIFA’s top team 12 times since the governing body began the rankings in 1993. Three ended in victory: Brazil in the 1998 Gold Cup semifinals in Los Angeles, Spain in the 2009 Confederations Cup semifinals in South Africa and at Germany in a friendly last year.

“He’s probably the best of all time,” Bradley said of Messi. “But there are still plenty of examples of days when teams can collectively make the game hard on him and make space tight, and force him to have to dribble sideways in moments, to put him on his right foot, to eliminate certain things, and to have the mentality and commitment to do it over and over and over again for 90 minutes.”

In this Copa America, Messi sat out the group opener with a sore back and played a combined 74 minutes in the next two matches, notching a 19-minute hat trick against Panama before going 90 against Venezuela.

“We are ready to bite, to fight, to chase them, to be all over them,” Klinsmann said. “If we repeat that and add a couple more percent to it, it’s going to be fun. We have to be on top of our game defensively – that’s a no-brainer – but we’ve done extremely well in the whole competition defensively.”

The Americans have yet to concede a goal in the run of play.The goals against the United States have come off a corner kick, penalty kick and free kick.

“We are 100 percent going in with the belief that we can play with them,” defender Matt Besler said. “We’ll see on Tuesday night. But you can’t go into a game conceding that belief already.”