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Jurgen Klinsmann's side will be kicking themselves about the manner in which they fell behind when, in the fifth minute, a seemingly innocuous cross skewed off the shin of Stoke City defender Geoff Cameron and the Manchester United winger gratefully accepted the chance to score from close range.
However, the Stars and Stripes responded well after conceding early and were nearly back on terms by the 13th minute when Clint Dempsey's free kick was only narrowly over the crossbar with keeper Beto a helpless spectator.
The Seleccao's bad luck with injuries continued when striker Helder Postiga was forced off through injury with little more than a quarter of an hour played in Manaus.
Dempsey was soon involved in another US attack when he burst down the right-hand side, before unleashing a shot from a tight angle that Beto needed to be equal to.
The Seattle Sounders forward continued to threaten in advanced areas and came close to an equaliser in the 18th minute as his attempted lob was narrowly wide.
Portugal struggled to resist their opponents in forward areas and Dempsey again proved difficult to contain as he found Michael Bradley, but the midfielder's scuffed shot drifted narrowly wide of the post.
With 10 minutes of the half remaining, Jermaine Jones then tried his luck from range and Beto needed to get down well to keep his effort out.