For more than 39 minutes, the nation’s most celebrated teenaged basketball prospect demonstrated why dozens of NBA scouts flocked to Atlanta to see him play.

Only in the final 20 seconds of Duke’s 77-72 loss to Kentucky did 17-year-old Cooper Flagg finally act his age.

With Duke’s nine-point second-half lead gone and the score now tied, Flagg seemed to sense it was his moment. He attacked 6-foot-11 Andrew Sarr off the dribble, used his body to create space and then started to rise for a mid-range jumper.

Everything was set up for Flagg to deliver a signature basket on the Champions Classic stage, but for one of the first times all night the Duke freshman failed to come through. He never saw Otega Oweh leave his man, enabling the Kentucky guard to poke the ball away, streak down court and draw a foul as he went up for a potential go-ahead layup.

Flagg had a chance for redemption after Oweh sank two foul shots, but the Duke freshman lost control of the ball as he tried to take Kentucky 7-footer Amari Williams off the dribble. He lay flat on his back in disbelief until two teammates came over to help him up.

That would have been painful enough for Flagg, but it somehow got worse when Kentucky’s Lamont Butler went to the foul line to try to clinch the victory with 5.1 seconds remaining. Flagg failed to box out Oweh after Butler missed the second of two free throws, squandering Duke’s chance at a last-gasp 3-pointer to tie the game.

Flagg played all 20 minutes after halftime and finished with a game-high 26 points and 12 rebounds. He expressed gratitude that Duke coach Jon Scheyer trusted him down the stretch while lamenting that, “It didn’t work out.”

When asked if he felt fatigued late in the second half, Flagg admitted, “I definitely did a little bit, but I tried to fight through it as best I could.”

If Flagg’s mixed results in his first big test was the biggest story coming out of Tuesday’s game, then close behind was Kentucky earning first-year coach Mark Pope his first signature victory. The 19th-ranked Wildcats rallied from a nine-point second-half deficit behind balanced offense and improved defense.

When John Calipari fled to Arkansas on the eve of last season’s national title game, Kentucky athletic director Mitch Barnhart responded by taking two big swings. He made Dan Hurley tell him no, and the UConn coach quickly elected to chase a three-peat in Storrs. Then he offered the job to Scott Drew and flew his family to Lexington, only to have the Baylor coach decide to pass on the opportunity.

The not-so-splashy “Plan C” that Barnhart pivoted to has so far proven to be an inspired third choice. Pope is one of Kentucky’s own, a team captain on Rick Pitino’s powerhouse 1996 national championship team and the first ex-Wildcat to coach his alma mater since 1985. He’s also the anti-Calipari in many ways, an innovator who runs a modern, 3-point-heavy offense.

The veteran-heavy roster that Pope assembled on the fly bears little resemblance to Calipari’s freshman-laden teams. The Wildcats feature nine transfers, including six fifth-year seniors. Their players have made 586 combined starts even if they had little time playing together.

Experience mattered Tuesday night when Duke extended its lead to 56-47 with 13 minutes remaining in the second half. Instead of wilting, Kentucky ratcheted up its defensive intensity, allowing the Blue Devils only five made field goals the rest of the way.

Tyrese Proctor and Kon Knueppel were strong in support of Flagg in the first half, but both withered in the face of relentless Kentucky pressure after halftime. That left Flagg to be Duke’s primary initiator and playmaker, a lot to ask of a 6-foot-9 teenager playing in his first marquee collegiate game.

For awhile, Flagg flourished. He shot 9-for-19 from the field, made smart decisions with the ball in his hands and showcased the athleticism, versatility and competitive fire that make him the presumed No. 1 pick in next year’s NBA Draft.

On one possession, he blocked a shot, led the fast break and drew a foul at the other end. On another, he outraced every slow-to-react Kentucky transition defender the length of the floor, his hustle producing a 2-on-1 fast break, an uncontested dunk and an immediate Kentucky timeout.

“He’s got to touch it,” Scheyer said. “I wish you could say that every time it’s going to work out, but that’s not reality. I’m just so proud of him. He willed us in that game, especially down the stretch.”

For Flagg, the final seconds were a disappointing ending.

All too abruptly, a signature performance became a learning experience.