The Tonga captain celebrated turning 34 on Sunday by scoring a try and slotting over the first conversion he has ever taken in a man-of-the-match performance at Hanazono Rugby Stadium.
A first win of the tournament ensured it was the USA rather than Tonga who finished bottom of Pool C.
Piutau was carried off the field by his team-mates after the game before paying tribute to his sister Ema, who lost her battle with stomach cancer in April.
"It’s been a tough journey, but all this has been done in her memory." said Piutau.