Portugal coach Antonio Oliveira has admitted his side were outperformed physically by the USA in their surprise 3-2 defeat.
However, playmaker Luis Figo contradicted his coach's assessment by pointing out that Portugal dominated the second half without being able to make the vital breakthrough.
Such a performance after the break would not have been necessary had their early defending and the form of goalkeeper Vitor Baia not been so abysmal.
The USA had effectively sewn up the points after 36 minutes by which time they were 3-0 up.
The USA's John O'Brien opened the scoring after three minutes before an own goal from Jorge Costa doubled the advantage on 29 minutes.
Striker Brian McBride scored the third before Portugal clawed back goals from Beto and an own-goal from Jeff Ago to set up a thrilling last quarter.
But Portugal - spearheaded by World Player of the Year Figo and pre-tournament favourites to win the group - were kept out by a well-prepared US team.
"The USA played very well. They were very physical and they made
it very difficult for us," said Oliveira, who bemoaned his country's lack of time together before the tournament.
"We had only two weeks of preparation - that is not an excuse, that's a fact.
"But they played very strongly and we have a lot of problems imposing our game.
"We displayed a lot of frailty at the back. This complicates things for us but all is not lost.
"We created a lot of chances to score but for one reason or another the ball didn't go into the net.
"We didn't have much luck but that is no excuse. We tried everything to get at least a draw and it goes to show what I've always said - the World Cup is a very tough competition."
Figo agreed with his manager in his view that the Euro 2000 semi-finalists threw everything at the States in the second half, but disagreed that they were outmuscled by Bruce Arena's team.
"If we were physically inferior we wouldn't have had the ball in the second half as we did," the Real Madrid player said.
"To start a World Cup with a defeat is an awful result, but all was against us and we conceded some very strange goals," he added.
Portugal's next match is against Poland in Jeonju on 10 June.
USA and South Korea lead Group A with three points, while Poland and Portugal are pointless.