The United States beat Portugal 22-17 with two second-half tries in four minutes then hung on while a man down at Estadio Universitario on Saturday.
The Eagles led 10-6 at half-time thanks to a try by star winger Takudzwa Ngwenya, who broke clear then ran over a defender.
After Portugal fly-half Joseph Gardner closed the gap to one with his third penalty, America struck quickly twice, with midfielder Andrew Suniula sliding in in the 56th minute, followed four minutes later by a rampant charge to the line by veteran replacement Paul Emerick.
The Eagles enjoyed most of the possession in the second half, but after replacement number eight Jonathan Gaganio was sin-binned, Portugal flanker Vasco Uva earned a try and gave the match a tense finish.
Pens: Gardner 4
For United States:
Tries: Ngwenya, Suniula, Emerick
Cons: Malifa 2