Jayson Tatum and Luke Kennard combine for 49 points as Louisville fails to connect on a game-tying three and Duke advances to face North Carolina in the ACC semifinals. (0:34)

NEW YORK -- David Levitch, a generously listed 6-foot-3, 180-pound twig of a player who is so rail-thin his shorts need an extra roll down to stay attached to his hips, does not look the type to figure into a basketball kerfuffle.

But Levitch was defending Grayson Allen, and things seem to happen when Allen is involved.

What happened between the ACC tournament quarterfinal between Levitch and Allen not only changed the tenor of the game between Louisville and Duke, it might just change how this season ends for the Blue Devils.

Because while Grayson Allen fatigue might be real and is thoroughly understandable, the simple truth is Duke will go as their star-crossed junior goes. When he is good, the Blue Devils are very, very good, and when he is bad, they are ... perhaps not horrid, but certainly average.

Allen was better than good against the Cardinals, his 18 points fueling a ferocious second-half rally that left Louisville flat-footed, stunned and ultimately packing, losing 81-77.

Grayson Allen's 18 points off the bench helped Duke sink Louisville on Thursday and earn a spot in the ACC tournament semifinals. Brad Penner/USA TODAY Sports

Of course, this being Allen, he also was controversial, swishing three free throws that were either merited after Levitch landed on him or gifts after an Oscar-deserving flop, depending on your team allegiance.

This is who Allen is now, or at least who’s he been caricatured into, and Duke’s postseason fate hinges on his ability to ignore the drama and simply be.

“Look, I love Grayson. Grayson, I got his back all the time," said Mike Krzyzewski, whose Blue Devils move on to face rival North Carolina in a semifinal Friday. “And everyone in our program has his back all the time. We're just ... the public eye on our program is a blessing and can be a curse. So we have to be able to deal with all of it. The thing in dealing with all of it is for everyone to know that we're together. I believe in him. I love him. And I thought what he did today was sensational. I loved it. I loved it. He was himself today."

That’s been the thing all season, really: Allen’s ability to be himself, to get out of his own head and simply play. He has succeeded, failed, succeeded and failed again in the span of one season, his roller-coaster ride coinciding almost in lockstep with Duke’s fortunes. Good Grayson equals a big win against North Carolina. Bad Grayson turns into a loss to Syracuse.

Dogged by critics and hampered by injuries, he insists his confidence has never flagged, but his game certainly has.

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Against Clemson just 24 hours earlier, he put up his first goose egg since the 2015 Elite Eight, playing a spare 12 minutes and drawing a technical for an overly emotional reaction to what he thought was a bad foul call.

It seemed fair, then, to question not just whether Allen's balky ankle was worse than it seemed but if his psyche was really more the issue.

His teammates talked to him after the lousy night. Allen didn’t necessarily need the bucking up -- “He’s a fighter, a tough kid,’’ sophomore Luke Kennard said -- but they figured he could use at least the affirmation.

“I reminded him that when we’re special, it’s because of him,’’ senior Amile Jefferson said. “When he’s on, he makes our team better. But he’s a player, and players understand the game. What matters is how you respond.’’

Allen said his response Thursday wasn’t that complicated. “I just attacked the same way and made some shots today. That was really it,’’ he said.

Really, though, it’s all a lot more than that. Once a national player of the year candidate, Allen now is relegated to sixth man, returning to his freshman-season roots. He is back to being a spark plug off the bench, trying to infuse his team much as he did in that memorable 2015 national championship game. He knew his teammates were tired, or more, exhausted. The Blue Devils’ starters all play yeoman’s minutes, and since Allen barely broke a sweat against the Tigers, he was ready Thursday.

But this was about more than making shots and scoring points. Allen also doused the flames instead of fanning them during the most crucial juncture of the game.

The Blue Devils simply could not stop the Cardinals on the way to the rim, Louisville building up a 10-point lead by simply connecting layups to dunks to floaters.

A desperate Krzyzewski went zone -- “any port in the storm,’’ he said by way of explanation -- and the defensive switch slowed the Cards’ momentum.

What really stopped Louisville in its tracks was Allen.

He and Levitch tangled on a drive to the hoop, with Levitch whistled for the foul. The two emerged from behind the stanchion, and with the crowd in full throat, Levitch sneered a bit while Allen plaintively held up his hands as if to say, "Nothing to see here.’’

Three seconds later, Levitch pancaked -- or Allen flopped -- Allen on a deep corner 3. Barclays Center erupted, one bench jumping up to cheer, the other to protest, one fan group screaming and one celebrating.

Amid the chaos, Allen stepped to the free throw line and drained all three shots.

Game on, and essentially, game over.

“Well, for me, energy produces energy,’’ Krzyzewski said. “I think we already had energy, and people saw it. And then Grayson has played for three years all-out. All-out. All-out. That was another example of it, and his teammates and everyone responded. And the people who don't want us to win, they responded. God bless everyone. But energy produced more energy.’’

For Duke, energy comes in the form of the player who wears jersey No. 3.

Because when Allen brings it, Duke can beat anybody.