The Gamecocks (24-10) seem capable of anything after this one.
Thornwell had 24 points, Chris Silva scored 13 of his 17 points in the second half and Duane Notice had 17 points, 14 in a 65-point second half, as South Carolina rallied from 10 points down for a second straight NCAA victory.
The 65 points were the most given up in a half by a Mike Krzyzewski-coached Duke team.
The Gamecocks shook off a 20-percent shooting first half for the win.
"I told the guys at halftime, someone's got to have the courage and make shots," South Carolina coach Frank Martin said.
When the horn sounded, South Carolina players, coaches and staff rushed to the fans at Bon Secours Wellness Arena to celebrate — a crowd that included Houston Texans Pro Bowl defensive end Jadeveon Clowney, a former Gamecock.
Next up is the East Regional where South Carolina takes on third-seeded Baylor, an 82-78 winner over Southern Cal earlier Sunday.
Duke (28-9) was attempting to reach the round of 16 for the sixth time in eight seasons. The Blue Devils, though, could not surmount South Carolina's stifling defense. Leading scorer Luke Kennard had his second straight subpar shooting game, finishing 1 of 6 for 11 points before fouling out.
The Blue Devils made five of their eight 3-point attempts in the first half, yet only 5 of 19 after the break. They had tied a season high with 18 turnovers against the relentless Gamecocks attack.
Silva added 10 rebounds for his fourth double-double this season.
Grayson Allen led Duke with 20 points.
Krzyzewski said South Carolina's physical nature wore down his team in the second half.
"That's the most physical team we've faced all year," he said.
Even as South Carolina got in front, Duke believed it could mount a comeback like the several it pulled off last week to become the first ACC champion to win four games on the way to a title.