US coach Jurgen Klinsmann pointed to Greece's shock Euro 2004 victory as a reason for optimism at the World Cup in Brazil.

The USA have been placed in a tough group alongside Ghana, Portugal and three-time winners Germany, but Klinsmann refused to rule out the possibility of his side going all the way in the competition.

"Talking about winning a World Cup is just not realistic. But Greece in 2004, nobody said they would win the European Cup, but they did," said the former Germany coach.

"Football is not predictable. If we get through this group, the sky is the limit."

After their opening game with Ghana on Monday, USA face Portugal before their final group game with Klinsmann's homeland, but the 49-year-old rejects the tag of outsiders which is being labelled at his side.

"You have to be realistic," claimed Klinsmann.

"We are making every year another step forward. We don't look (at) ourselves as an underdog, even if a lot of the others do."

Despite his optimism, however, Klinsmann is aware of the quality of the other sides in the group, including their first opponents Ghana, who beat the USA at the last two World Cups.

"This is probably the most difficult group in the World Cup. We know about their strengths, but we know their weaknesses.

"Ghana is the number one or two in Africa."