The Americans were seen as favourites following their brave performance against the world champions four days earlier but they looked to be suffering from the after-effects of that clash as they got off to the worst possible start.
They conceded a try after just 72 seconds and found themselves trailing 13-0 after 26 minutes before going on to enjoy long periods of domination with little reward in a 25-15 defeat.
“People might think our performance against England had an effect, but we won’t be using that excuse,” insisted flanker Louis Stanfill, one of their two try scorers.
“That’s an easy excuse to use.”
Former Tonga captain Viliami Vaki came off the bench to score the clinching try and get his side’s campaign off to a dream start.
The veteran forward pounced for a try 11 minutes from the end to kill off the brave efforts of the Eagles and claim Tonga’s third victory in World Cup history, their first for eight years.
Tonga fly-half Pierre Hola, who kicked four goals from five attempts, admitted his side had been helped in their preparations by studying their opponents’ 28-10 defeat by England.
“We prepared well,” he said.
“We watched the USA play England and that helped us to plan our strategy and we got a good start. We were on a roll.”
Flanker Finau Maka was driven over for the game’s first try and Hola added the conversion before punishing American indiscipline with a couple of penalty goals.
The United States struggled in the set-piece play and found themselves with their hands full against the combative Pacific Islanders’ forwards, but gradually weathered the storm.
Guided by the experience of fly-half Mike Hercus, they made every effort to bring lively left winger Takudzwa Ngwenya into play but they only had a Hercus penalty to show for their efforts before the Americans trailed 13-3 at half-time.
However, the Americans were the better side for long periods of the second half and twice got back to within a score of their opponents.
Leeds prop Mike MacDonald took Hercus’ neat pass to crash over in the corner on 46 minutes after hooker Owen Lentz had been held up over the line. The conversion was too far out for Hercus but, at 13-8, the momentum was clearly with the States.
However, a breakaway try halted the Americans dead in their tracks.
Tonga’s inspirational captain Nili Latu broke through the first line of defence from the halfway line and full-back Vungakoto Lilo took play on to get winger Joseph Vaka over for his first try for his country.
Stanfill gave the USA renewed hope with a converted try on 66 minutes that cut the deficit to three points, but Vaki struck the decisive blow and Hola kicked his fourth goal.
Tries: MacDonald, Stanfill. Cons: Hercus. Pens: Hercus.
Wyles, Sika, Tuipulotu, Esikia, Ngwenya, Hercus, Erskine, MacDonald, Lentz, Osentowski, Parker, Mangan, Stanfill, Clever, Bloomfield.
Eloff for Esikia (60), Moeakiola for MacDonald (54), Burdette for Lentz (49), Basauri for Bloomfield (55).
Tries: Maka, Vaka, Vaki. Cons: Hola 2. Pens: Hola 2.
Lilo, Tu’ifua, Hufanga, Taione, Vaka, Hola, Havea, S. Tonga’uiha, Lutui, Pulu, Fa’aoso, Hehea, T-Pole, Latu, Maka.
Tupou for Taione (72), Havili for Vaka (60), Tuipulotu for Havea (40), Toke for S. Tonga’uiha (66), Taukafa for Lutui (76), Vaki for Latu (65), Filipine for Maka (65).
S Dickinson (Australia).