First-half tries to front-rowers Lorenzo Cittadini and Leonardo Ghiraldini, as well as a penalty try awarded just after the hour, were enough to see the Azzurri to their third straight win.
There are much harder tests to come for the Italians - New Zealand and Australia will be their next two opponents and on this evidence they could be long afternoons.
Italy got off to an excellent start, crossing the line with only six minutes gone thanks to man-of-the-match Cittadini.
But Tonga - as always hugely physical in the tackle - were more than a match for their hosts and pulled into the lead through prop Alisona Taumalolo just after the half hour.
Ghiraldini grabbed the lead back for the hosts on the stroke of half-time, handing them an 18-16 advantage heading into the second half.
Italy thought they had the match sewn up when they were awarded a penalty try, converted by Kris Burton, just after the hour which extended their lead to 12 points.
However Jacques Brunel's side, who still cannot shake off their habit of making unforced errors, went to sleep and allowed Fetu'u Vainikolo to pounce onto a smart diagonal kick on 67 minutes to shave the deficit to only five points.
Italy managed to cling on to their lead however, and record a welcome confidence boost ahead of their mouth-watering challenges against the world champion All Blacks and Australia.
Cittadini said: "I'm happy - because above all we won, despite having committed a lot of errors, which we will work on in the week.
"We know that we have to do better next week, however I couldn't have hoped for better than this today."
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