The last match in Pool C may only decide fourth and fifth spots, but the standings belie these teams' performances.
Both gave France a fight. USA trailed 12-9 in Fukuoka with under 15 minutes to play, but fell away to a 33-9 defeat.
Tonga threatened to repeat their 2011 victory over Les Bleus. Though they went 17-0 behind, they were finally edged out by just two points, 23-21.
The Sea Eagles are perhaps therefore slight favourites.
They have also had a clear week to prepare, whereas USA must recover from a bruising encounter against Argentina on Wednesday. They take to the field with 12 of the 15 who kicked off that match.
Tonga, pictured performing the Sipi Tau before their game against Argentina, have won eight of the teams' previous nine meetings, their only defeat coming 20 years ago in San Francisco in their very first clash.
"When we stuck to what our plan was we were pretty good. When we went off script we weren't," said USA coach Gary Gold of the performances so far.
"That's been our biggest lesson in this Rugby World Cup. When that has happened, the calibre of the teams we're playing punish you."
Tonga's downfall has been their slow starts. As well as the France game, Argentina also took a 28-0 lead. Coach Toutai Kefu says they will try something a bit different this time.
Over-long warm-ups were one of the areas being looked at. Kefu also said: "We're going to try to connect the players earlier in the day and do a few new things in the changing room.
"We watched the Americans' game on Wednesday and they were very aggressive. If we don't bring anything but our best we're going to be in trouble."
Gold has highlighted the Tongans' physicality. He said, "If you consider it was only a few short weeks ago that they conceded 90-odd points against New Zealand (92-7 in their final warm-up match), it's massive credit to Toutai and his team how they've dusted themselves off and come back fighting."