DURHAM, N.C. (AP) -- With its star big man on the bench with foul trouble, Duke went small to beat its top rival - and one of its smallest players came up big.

The run that propelled the second-ranked Blue Devils to a 73-68 victory over North Carolina on Wednesday night started when Mason Plumlee sat down and backup guard Tyler Thornton warmed up.

''I think the hero for us this game was Thornton,'' coach Mike Krzyzewski said. ''He would not let us lose.''

Plumlee finished with his usual big numbers - 18 points, 11 rebounds - while Quinn Cook scored 18 points and Rasheed Sulaimon finished with 13 for the Blue Devils (22-2, 9-2 Atlantic Coast Conference).

But the contributions of Thornton couldn't be overlooked.

Thornton finished with nine points on three 3-pointers - or, as many points as he had in his previous three games combined - and two of those 3s came during the run midway through the second half that completely flipped the game's momentum.

''It's just the heart and the will to want to win,'' Thornton said. ''Especially coming down the stretch, you've got to go all out and leave it on the floor, and I think we did that.''

Duke shot 44 percent - 52 percent after halftime - and erased a slow start with that timely run and win its sixth straight this season and sixth in eight meetings in college basketball's fiercest rivalry.

P.J. Hairston matched a career high with 23 points and Reggie Bullock had 15 points with four 3-pointers for North Carolina (16-8, 6-5), which led for the first 26 minutes but went on to lose its second straight.

The Tar Heels were 13 of 23 from the free-throw line and missed 7 of 10 during a critical late stretch while falling to 1-4 this season against ranked opponents.

''If I knew how to fix the blessed thing, I would have fixed it,'' coach Roy Williams said of his team's struggles at the line. ''The bottom line is, we didn't make free throws today. We're not a good free-throw shooting team in games.''

Still, they trailed just 65-61 in the final minute and appeared to have gotten a stop by forcing Thornton to miss a long 3-pointer with the shot clock winding down. But Bullock fouled Sulaimon on the rebound, and the freshman hit both free throws with 37.5 seconds left.

Hairston hit a free throw on North Carolina's next possession to cut it to 67-62, but Plumlee countered with two free throws with 30.3 seconds left to make it a three-possession game.

Seth Curry scored 11 points in his sixth straight double-figure performance against North Carolina.

The win was a nice present for Krzyzewski, who was celebrating his 66th birthday.

And an unorthodox move - putting one of the best big men in the nation on the bench, however briefly - wound up putting Duke ahead for the first time in this one.

Plumlee picked up his third foul 31 seconds into the second half and uncorked an untimely 20-foot jumper a few minutes later, prompting Krzyzewski to burn a timeout. He went to a smaller lineup, sitting Plumlee in favor of two power forwards, Amile Jefferson and Josh Hairston.

''I thought he was playing like he had three fouls,'' Krzyzewski said. UNC's James Michael ''McAdoo was just going at him so that McAdoo was either going to score, or Mason was going to foul him.''

The move freed up some space for the Duke guards and immediately led to six quick points to start the 19-7 run that put the Blue Devils ahead to stay.

Duke outscored North Carolina 11-3 during the 4-minute stretch with Plumlee on the bench and took their first lead when Curry swished a 3 from in front of the bench to make it 42-41 with 14 minutes left.

Thornton - who had missed 12 of 14 3-pointers during his previous eight games - hit two of them from the same spot in the right corner, capping the spurt with his second that made it 50-45 with 12 1/2 minutes to go.

Curry eventually stretched the lead to 59-51 with another 3 with 5 minutes left.

''They started knocking down shots in the second half, open 3-pointers,'' Bullock said. ''I think our team played great, we played with better sense of urgency, we played with better effort, we were more involved in the game and what's happening. But they started knocking down shots and they started gaining momentum, and we wasn't connecting when we were open.''

Dexter Strickland added 14 points but McAdoo was held to nine on 4-of-12 shooting for North Carolina, which came in as a written-off, double-digit underdog after a 26-point loss at No. 3 Miami that marked its worst loss of the season.

But the Tar Heels were aggressive early and methodically built a double-figure lead, the third straight year they came into Cameron and went up by 10. Bullock's third 3 of the half with about 6:45 left put North Carolina up 28-18.

''The intensity tonight was better than it has been all year long,'' Williams said.