(Reuters) - The United States are full of confidence after a dramatic win over Ghana but coach Juergen Klinsmann remains wary of a Portuguese backlash after the Cristiano Ronaldo-led side were mauled in their World Cup opener by Germany.

A last-gasp 2-1 victory over Ghana showed the United States could keep cool in a big match and they will need to do so again when they take on world player of the year Ronaldo on Sunday.

A win over Portugal in the Group G contest in the jungle city of Manaus should put the Americans through to the last 16 for the fourth time in the last seven tournaments.

However, Klinsmann warned that Portugal's 4-0 thrashing by Germany could jolt them into action and the Americans will have to cope without burly striker Jozy Altidore, who suffered a hamstring injury against Ghana.

There was better news on the fitness of Matt Besler and Clint Dempsey, who Klinsmann said would both be fit for the Portugal game.

"With all the respect we have for the Portuguese team, it’s a very difficult situation for them after their defeat, they are with their backs against the wall," Klinsmann said after a training session in Sao Paulo on Tuesday.

"It makes it even more difficult to get a result, but that’s what a World Cup is all about."

DREADFUL DEFENDING

Poor finishing, dreadful defending and indiscipline combined to make it a miserable opening game for Paulo Bento's Portugal.

Five of their starting lineup against Germany were over 30, including three of the back four, so a more youthful U.S. team will aim to press the Portuguese in the hot and humid climate.

Portugal will also be without defender Pepe, whose red card for leaning into Germany striker Thomas Mueller with his head rules him out.

The match will be the second World Cup meeting between the teams and the United States will hope for a similar result to 2002 in South Korea when they shocked a more fancied Portuguese side 3-2 in the group stage.

Although the United States are no longer surprise packages, they will be without striker Altidore.

"The medical staff is doing a tremendous job, so we still have to hope the Jozy will be back," Klinsmann added. "How quickly? That is down to his healing process."

While Besler failed to start the second half due to a tight hamstring, Klinsmann said he was ready to go in the second group game.

Dempsey, who suffered a broken nose from taking a boot to the face shortly after scoring the fifth-fastest World Cup goal, will soldier on, perhaps with a mask.

"I don’t know how much a mask can protect you," added the German. "It was tricky during the game, he barely could breathe, he struggled with that, but once it's broken it's broken.

"Unfortunately it's also part of our game."

Klinsmann said it was important not to dwell on the success of the opening game and to look ahead.

"We only talk now about how we can beat the Portuguese team, with all these amazing players they have," he said.

"This is our goal. We believe we can go to Manaus and beat them. We want to get to the next phase and we need to beat Portugal."