LAS VEGAS — First, there were whispers, which turned to murmurs, which then elevated to a full-on buzz. 

The public address announcer at the Thomas and Mack Center called it “The Main Event,” but the previous game was getting in the way. Late fouls prevented the debut of Cooper Flagg and reintroduction of Bronny James by a handful of minutes, prompting boos from the crowd. But once the bell rung, Flagg delivered beyond what the numbers illustrated. The No. 1 pick in last month’s draft had some electrifying moments in his first taste of NBA life, while James looked more comfortable in a professional setting than he did last season — albeit in the rough-and-tumble setting of Summer League.

Flagg, who ran point guard at Mavericks coach Jason Kidd’s urging, handled the Lakers picking him up at full court and bumping him when he tried to use his 6-foot-8 frame to get in the low post. He also found his athleticism translates even in the physical nature of Summer League, getting out in the open floor for an easy dunk and even trying a poster in traffic on Christian Koloko that came up short.

The conversion or failure wasn’t so much the thing. It was the audacity, the bounce.

It didn’t matter that he looked a bit parched as much as he was winded, taking 21 shots and not making a field goal in the second half of Dallas’ 87-85 win. He just looked like he belonged out there, and the gifts were evident.

For context, he took 22 shots just one time in 39 games at Duke. On Thursday, he finished with 10 points, 6 rebounds, 4 assists, 3 steals and 1 block — the block being a spectacular one in the game’s final minutes, leading to the go-ahead triple that kept the Mavericks up for good.

“When you get that ball in your hands, the conditioning aspect comes into play,” Mavericks summer league coach Josh Broghamer said. “They were picking them up full court. They were trapping them. They're being super physical, attention with him in the post, every time he drove it.”

The most telling physical exchange came with James bodying him up on consecutive plays, giving the crowd what they wanted to see. Despite his height advantage, Flagg couldn’t move James, and James poked the ball away but was called for a foul.

Quick learner that he is, Flagg then caught the inbounds pass, recognized James was six inches shorter, rose and fired a true 21-footer on the left wing — much to the delight of his future teammates on the sideline, one being All-Star Kyrie Irving.

“I was excited. I was a little nervous in the environment,” Flagg said. “But I was just happy to be here.

“It was fun, high energy, a lot of fans. Everybody showed up. It was an intense energy. That’s how you look forward to it, that’s why we play, those types of moments, those type of games.”

If everyone could’ve spread out and watched Flagg and James go at it all game long, the crowd would’ve been satisfied. 

James had his moments, too, and they were early — just like Flagg’s. He hit two jumpers in the first two possessions with Flagg guarding him, one being a triple, to give this atmosphere even more of an AAU-buzz.

“He’s got, like, half a foot on me,” James said. “He’s great. I watched him in college, amazing player.”

Of the moment, Flagg said: “I don't know how tall he is, but a smaller guard like that on me, I gotta try and take advantage of (the) mismatch and the size advantage. So it wasn't anything more than that, but it was fun.”

James still played mostly off the ball and, to his reputation, was looking for his teammates in the crowd. He made some nifty passes and others were swallowed up by a quick defense. He didn’t get up as many shots, and didn’t hit another shot after those first two field goals. His last attempt, a step-back 3 with seconds left that could’ve won the game for the Lakers, went awry.

James, like Flagg, could very well be in college right now. Flagg reclassified to get to Duke early and won’t turn 19 until he has two months of real pro experience under his belt. James left USC early and could probably benefit with more reps at that level.

Instead, they’re in the big leagues figuring it out with the brightest lights the NBA can offer in the summer.

Both had considerable rust. James didn’t play big minutes in the California Classic last week and was ramping up to this showdown, finishing at 20 minutes. Flagg had been the assumed No. 1 pick for the last 12 months, so he didn’t have to do much by way of workouts following his college season at Duke.

It feels unlikely Flagg will go through the full slate of games in summer league. He got his taste of the NBA, and so did the NBA, getting its first look at, arguably, the next big thing.

“I haven't probably taken that many shots before, so that's somewhat new,” Flagg said. “I'm gonna try and figure it out, but new experience. So I'm excited to keep playing move forward.”