The curtains were drawn so no one could peek into the Hub 925 gym 20 miles east of Oracle Arena. Outside of a handful of select staffers in the room, no one knew about this secret workout in late August.

This was the day that Stephen Curry would train with Luka Doncic, perhaps the most hyped European prospect ever. There was no live stream on Instagram, no curated House of Highlights sizzle reel.

Doncic’s agent, Bill Duffy, wanted to keep it quiet. Only a small circle -- maybe a dozen people -- attended, but hardly anyone outside that Pleasanton, Calif., gym had a clue that the two-time MVP was going to work out with the Slovenian prodigy. It was so hush-hush that not even Mavericks owner Mark Cuban knew about it.

“For me, I just wanted him to be exposed to the excellence of Steph,” Duffy told NBCSports.com. “Not just Steph’s skill, but appreciating the work that goes into it.”

Curry had already scheduled a workout, led by his longtime trainer Brandon Payne of Accelerate Basketball, with high school phenom Jalen Green, who could be the No. 1 prospect in the class of 2020. After a call from Duffy, Payne made a slight change of plans.

Luka is going to be in town for a day or two. Can he join Steph for a workout?

Of course, Payne told him, but there was something Duffy thought Payne should know.

“Hey, you know, he’s never really worked out like this before,” Duffy told Payne over the phone. “All of this is going to be new to him. Be patient with him.”

As a teenager, Doncic had already dominated the pro leagues in Europe, earning both the EuroLeague MVP and ACB MVP in 2018, but he’d never worked out with a player of Curry’s caliber. He had done shooting sessions with shot guru Mike Penberthy, now with the New Orleans Pelicans. But nothing quite like what he was about to experience.

In NBA circles, Curry’s workouts are the stuff of legend. How else does one go from Davidson to the NBA’s first unanimous MVP?

And so, Doncic tried Curry’s workout, which was supposed to last an hour. It lasted three.

“Luka,” Payne says now after working with Doncic up close, “is a basketball savant.”

* * *

It’s a Friday afternoon in Fort Mill, SC., at Payne’s training gym just outside of Curry’s hometown of Charlotte. Curry’s longtime trainer is cackling as the film begins.

“He’s f’ing huge,” Payne says of Doncic, who dwarfs Curry on the screen. “Luka is a big dude. He’s 6-8, easy. Big, big kid.”

The third round of 2019 All-Star voting returns had just come in and the big kid had widened his lead over Curry as the second-most popular player in the Western Conference (LeBron James is still first). Doncic, the only rookie among the top-40 vote-getters, received more love from voters than Curry, Kevin Durant and Kyrie Irving.

In the roughly seven months since the Mavericks traded up to pick him No. 3 overall in the 2018 NBA draft, Doncic has become a cult-like figure in league circles. His LeBron-like rookie averages of 19.9 points, 5.3 assists and 6.1 free throw attempts helped certify his starpower. The Ringer’s hypnotic #HalleLuka anthem only added to his aura.

In August, Payne saw some of these clues firsthand. He invited me to his film room to show me just before his flight to Los Angeles to launch Curry’s Underrated tour. He insisted that I see how far Doncic has come in just five months.

Payne has micromanaged Curry’s every movement using tools like strobe-light glasses, tennis balls and FitLight training bulbs that operate like traffic signals. He even trained Curry’s breathing patterns. Some have called him a basketball version of J.K. Simmons’ character from Whiplash.

Not my tempo.

Duffy felt Doncic would benefit from seeing what life is like at the mountaintop.

“I wanted to show him that someone at that level can take instruction too, be corrected,” Duffy said. “Brandon is up here telling Steph, do this, don’t do that, correcting him. I think someone like Luka, was like, ‘Woah -- Steph is top 2-3 players in basketball and he has someone pushing him to be the best he can be?’ To be exposed to that, was special.”

On the film, Payne is belting out dribble-and-shoot orders to both Curry and Doncic. One dribble, side step 2. One dribble, escape 3, coming back the other way! Side step 2, mirrored footwork, escape 3. Side step 2, come back, one dribble, escape 3.

“These are really difficult concepts,” Payne says. “When you’re Stephen, these are difficult concepts. This wasn’t a BS workout. This was a real I-have-something-that-I-have-to-get-done-with-Stephen workout. To be thrown into that and respond as well as he did, it was impressive.”

This particular drill took Curry three minutes flat to complete. His shots were flawless, barely grazing the rim. Doncic, trying to keep up with the commands and pace, took nine minutes and forty-eight seconds. (Doncic is also still learning English, his fourth language.)

“That’s a lot of information coming at him at once, that’s tough,” Payne says. “My terminology runs deep. The words aren’t complex. But when you start putting together combinations, it’s … tough.”

At one point during the workout, Doncic is seen dribbling up the court with two basketballs, a standard drill here in the States. Doncic is struggling. Jalen Green and Curry are just about lapping him. Doncic sticks out his tongue and laughs at it all. He does this a lot when he lags behind. I take this as a sign of Doncic’s self-awareness and his love for the game.

Payne nods. “Stephen’s like that, too.”

Then Payne grows serious again.

“Wait, back that up,” Payne says to his assistant at the controls. She rewinds and hits play.

“You see Luka’s closed lips, then you see his tongue out?” Payne asks me.

Yes, he’s a kid having fun. He’s 19 years old.

“That’s the first sign of neurological overload,” Payne corrects me. “Like, it’s too much for a player. He couldn’t breathe because his mouth is closed.”

He tells me to watch Curry -- note his open mouth as he dribbles down the floor, like the grille on the nose of a Ferrari, cooling the engine.

“So,” Payne explains, “those are the things I’m looking for.”

Something I’ve never considered: How good will Doncic be when he learns how to breathe properly?

Doncic was a tad beefy during this workout. He had just taken a month off to recover from a grueling year in Europe that saw him play about 90 games. Says Duffy: “We had to rest him.”

Payne estimates that Doncic has already shed 10 to 15 pounds this season, but the rookie of the year frontrunner is still far from slim. Listed at 218 pounds, Doncic’s conditioning was a point of pre-draft contention. What those people didn’t realize was how he uses his weight as a weapon. Doncic lulls you into thinking he can’t move quickly at his size before finishing opponents off.

“Everyone’s like, he’s got this stepback,” Payne says. “Yeah, he’s got a stepback, but you’re not looking at how he’s slowing them down before that stepback. You fall asleep, your feet die and then all of a sudden, he hits you with this real hard stepback. It’s so easy for him because he’s big and strong. His mechanics are really good, too.”

The week before the Curry workout, Doncic had spent time assessing and training his biomechanics at P3 Sports Science lab in Santa Barbara. They found that his deceleration abilities -- to figuratively go from 60-to-zero -- are some of the best they’ve ever seen among hundreds of pro athletes. (Doncic has visited P3 annually since he was 15 years old).

“He’s a gamer,” Payne says. “I have never seen anybody control the defender with change of pace the way he does. He takes control of you. You don’t control him. He controls you. He just has a touch and feel that you can’t teach.”

Beyond his physical attributes, Payne won’t stop talking about Doncic’s palpable enthusiasm for the game and how that propelled him through Curry’s relentless shot-making. Doncic did break out giggling several times during the grueling workout, partly in awe of Curry’s power and partly in making light of his own struggles.

By the end of the three hours, he was hanging with Curry shot-for-shot.

“Stephen has been working out with me as long as Luka has been playing basketball,” Payne says. “Stephen goes on these runs where he’s not human. I can’t imagine what it’d be like to be in that workout. It’s almost like when Stephen touches the rim, we don’t count it as a make.”

After the first hour of nonstop shooting and dribbling drills, Doncic wanted more. After hour two, Payne offered to end the workout. Doncic waved him off. More. After three hours, they went upstairs to lift.

“He’s smiling, laughing, having a good time,” Payne says. “I think he’s just having a good time doing it. He felt challenged. Stephen just did it. Now, I want to beat him.”

Footage from the August workout is a study in contrasts. Curry is a 30-year-old robot of precision and efficiency. Doncic, on the other hand, seems like a 19-year-old lump of clay -- really high-quality clay. And that’s the scary part.

“His footwork now is incredible, it’s amazing,” Payne says. “At the end of games? It is incredible. But if you think about it, it’s completely unrefined. Think about how good he’s going to be if he does get this stuff.”

* * *

It’s admittedly hard to watch Curry and Doncic play basketball next to each other and not immediately want to jump in a time machine. Is this what Doncic will look like in a decade? Curry is a model of efficiency both in his movement and shooting percentages. He hardly missed during the entire workout and never lost control of the ball. Doncic, however, was a work in progress. A dribble off his foot here, an errant jumper there. But after just a few repetitions, Doncic would become shockingly proficient, a trait that’s been evident during his first season in the league.

Doncic has scored 74 points in 76 minutes of clutch situations this season while shooting 51 percent in these close-and-late situations. After adjusting for pace, Doncic is scoring 46.8 per 100 possessions in the clutch, the ninth-highest rate in the NBA. He once went on an 11-0 run down the stretch to beat the Houston Rockets, prompting Mavs coach Rick Carlisle to tell ESPN, “It’s pretty clear he’s got a flair for the moment. He’s unafraid.”

Take for instance the “3-2-1 Perfect” drill, which Payne purposefully picked as one of the last drills to simulate end-of-game fatigue. Make three shots in a row from five different spots around the halfcourt. Then, come back and make two in a row at the same five spots in reverse order. Then, finish with a perfect five-for-five at one spot. The drill doesn’t end until the player makes the five straight.

“Luka’s quads are on fire right now,” Payne says watching Doncic labor through the drill.

On the film, Payne could be seen saying something to Doncic. What’d he say?

“I just asked him if he wants to quit,” Payne says. “He said no. Nine minutes is a long time to be shooting and doing hard dribbles non-stop.

“Damn this was hard. This was real hard, now that I’m watching it.”

Doncic hits the five and smiles, just like he did in December after hitting an impossible, buzzer-beating, over-the-backboard corner 3 against Portland to send it into overtime. This ability to perform fatigued and under pressure has fueled Doncic throughout his rookie season. When the moments are biggest, when the legs are their most tired, he rises to the occasion, usually with a grin.

* * *

There’s one moment during the film where Payne pauses. He wants to make sure I pay attention. Curry pulls Doncic aside and starts coaching him up, directing him where to go and helping him understand the drill.

“That’s the first time I’ve ever seen Stephen step into that mentor role in the middle of a workout,” Payne tells me. “That’s the first time.”

Curry does this at camps with youngsters, but never with a pro. Maybe Curry sees that Doncic is both.

Doncic is absorbing the NBA life and thriving. Where other top rookies like Deandre Ayton, Jaren Jackson Jr., and Trae Young have seen their scoring opportunities plateau, Doncic has raised his usage rate in each month, shouldering more and more of Dallas’ scoring load. In October, he ranked 67th in usage rate, per NBA.com tracking. But in January, his usage rate ranks 15th, ahead of stars like Curry, Damian Lillard and Kawhi Leonard. Doncic is only a rookie by definition.

“He’s made for this,” Duffy says of Doncic. “I don’t know what the ceiling is. Does Steph Curry put a ceiling on himself? Does Steve Nash? Kobe Bryant? Luka is definitely in that echelon.”

Cuban agrees, we’re just seeing the beginning.

“The best part of Luka, beyond how much fun he has, is that he has a lot of places he can improve his game,” Cuban says. “And he will work on them all.”

At the Accelerate offices outside Charlotte, Payne looks at his watch and sees he’s late for the airport. He quickly shows me a spreadsheet where they’re tracking every one of Doncic’s shots and pairing it with the drills he did in August. Doncic’s shooting percentages on Escape Dribbles, Wing Curl 2s, Slide and Read, all listed. Each shot has a link to both the in-game video and the workout video from August.

“I could watch this stuff all day,” Payne says as he grabs his bags. “There’s a whole lot of meat left on that bone, man. A whole lot.”