How the ultra-competitive Joe Mazzulla is challenging the doubters -- and what it means for the C...

Boston Celtics players couldn't help but give each other knowing smiles this week when first-year coach Joe Mazzulla had his drop-the-mic moment.

For three years, Mazzulla was one of those assistant coaches who built deep relationships with players. His job in Boston was mostly player development, which meant hours of one-on-one work after practices and before games that created bonds.

It also meant that, away from the gym, he was in the card games and the board games and, perhaps most intensely, the ping pong games. He was involved in a way head coaches and veteran assistants usually aren't with players.

And so the players and staff members who know him well weren't surprised when he fired back at critics.

"What, no one wants to ask about the adjustments we made from Game 1 to Game 2?" Mazzulla snapped as he walked off the podium following the Celtics' 34-point win.

Mazzulla -- the youngest coach in the NBA at 34 -- was fiery in response to the notion that he'd been outcoached by Philadelphia's future Hall of Fame coach Doc Rivers in Game 1.

"So this is the thing, he's an ultimate competitor and he wants to compete in everything he does," Celtics forward Grant Williams told ESPN after Boston secured its second straight win in the series on Friday to take a 2-1 lead going into Sunday's Game 4 (3:30 p.m. ET, ESPN).

"He's going to give it back if it's given. Some people will call that immaturity, other people will call that confidence, and I think it's the latter," Williams said.

Here's another thing: Rivers did outmaneuver Mazzulla in Game 1 when he used James Harden in a throwback role, putting the ball in his hands and creating isolation opportunities as the central part of the game plan, savaging the Celtics' defensive strategy against him on the way to 45 points and a Sixers upset win.

And now another: Mazzulla, facing one of the biggest challenges of his coaching career to this point, returned serve. His moves in the coaching chess game in this high-stakes matchup against a major rival -- putting Jaylen Brown on Harden and increasing ball pressure, using creative double teams on Joel Embiid since his return from injury, plus changes in pick-and-roll coverage -- have worked.

"Joe has far more pressure than I did my first year in the playoffs," said Rivers, referring to his 2001-02 Orlando Magic team that was a 7-seed. "He's doing a fantastic job. He's taking a team that went to the Finals last year. He's going to be the guy, just like me and just like all of us coaches, that you look at when anything goes wrong. And Joe's never experienced that, but that's just the way it is. It's part of our jobs. One thing I know about Joe, he knew that when he signed on. I guarantee it."

Boston has looked to cut off passing lanes for Harden when he comes off screens while still staying between him and the basket, encouraging him to take midrange jumpers. That, however, is not Harden's game and his indecisiveness has shown.

"I think [Mazzulla] handled it great, very professional," Celtics defensive ace Marcus Smart told ESPN. "He didn't go and try to argue. OK, he got outcoached. Now it's time to make adjustments. That's what the playoffs are about. And that's what a great coach, a great team and great players do."

Over the past two games, Harden and backcourt mate Tyrese Maxey have been severely limited by the Celtics' defensive changes. Harden hasn't been able to generate the space and separation against Brown and the supporting coverages, and is a woeful 5-of-28 shooting over that same time frame.

Maxey, who has had fewer transition opportunities to use his trademark speed and has struggled against larger defenders, is 10-of-30 shooting in the past two games. Even with Embiid looking better on an injured knee (he had 30 points Friday), the 76ers have averaged 94.5 points and have shot 39% in the series.

Mazzulla's nature as a competitor, one that is on center stage for the public in these playoffs, has very much been known to those around him for years, whether it was during his successful playing career at West Virginia or in the aggressive pickup games with assistant coaches he still partakes in after practices now.

And as the rest of the NBA is now learning, Mazzulla takes all of this seriously -- and much of it personally.

"It eats him up when something doesn't go well. It eats him up when he feels like he's let the team down," Celtics president Brad Stevens said earlier this year when he removed the "interim" tag from Mazzulla, which he had in the wake of his stunning handover from a suspended Ime Udoka last summer.

"He's not going to ever come in and say, I wish this person, or this person, or this person would've done their jobs better. He'll always say the opposite."

There will be more times when Mazzulla will be questioned. A coach's burden is that they're sometimes only one bad game from criticism, especially when their résumé and reputation are still very much being formed.

Mazzulla's task is even more complex. His relative youth -- starting center Al Horford is nearly three years older than him -- and the nature of the way he took the job under duress with a team that went to the NBA Finals last season and carried with it title expectations, magnifies everything.

There might be more chest-beating moments in the coming days, but there are sure to be more humbling times, too.

But Mazzulla was facing a rather big test after the Celtics fell down 1-0 in this series and he passed.

"It's a big moment. Playoffs are where coaches are made for a lot of people and for him being that first year with the reins as calling the shots, he handled himself very well and we're very proud of him," Smart said. "But it's not over and he understands that and we understand that."