Captain Sergio Parisse's third-minute try set the tone and, although Chris Wyles hit back, further scores from Luciano Orquera and Martin Castrogiovanni put the Azzurri in control at half-time.
A penalty try was the only score of the second half but it was sufficient to ensure the winner of Sunday's clash will progress along with Australia, assuming the Wallabies beat group minnows Russia on Saturday.
Parisse bettered his six-minute score against Russia after Italy made early inroads and the move ended with lock Cornelius van Zyl sending his skipper through a gaping gap and under the posts.
Tommaso Benvenuti was denied a second try when he was unable to ground a difficult bouncing ball and it was the Eagles who levelled instead, centre Paul Emerick breaking powerfully through the line and offloading for Wyles to touch down - and then convert.
Mirco Bergamasco and Wyles traded penalties before a rolling maul took Italy to the brink of the line and, after the ball was recycled, fly-half Orquera sliced through. Bergamasco's conversion attempt sailed wide.
The USA prevented Castrogiovanni grounding for a third Italian try, captain Todd Clever clashing with the prop on the ground as referee George Clancy referred the decision upstairs.
Wyles kept Luke McLean out after Benvenuti did superbly to keep the move alive but Castrogiovanni was virtually hauled over the line by Van Zyl to touch down. Bergamasco's missed conversion left the half-time score at 20-10.
The States started the second half positively but the impressive Emerick was just unable to stay in play after Mike Petri's kick deceived Bergamasco.
American flanker Louis Stanfill was sin-binned for a scrum infringement on the hour before Castrogiovanni was denied by the TMO once again after replays failed to show clearly whether the ball was grounded.
From the resulting scrum, though, Italy's dominance up front yielded a penalty try and a bonus point and Bergamasco easily slotted the kick.
McLean raced back to prevent replacement American full-back Blaine Scully touching down James Paterson's grubber but their was little else for Italy to worry about - though their tough battle for the bonus point will have encouraged the watching Irish.
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