CLEVELAND – From the bowels of the Quicken Loans Arena to the pressroom podium, people approached LeBron James about his team’s collapse, about his own collapse. These were mostly unheard-of topics during his stretch of three NBA championships and three Finals MVPs in five years, and this postseason had been a dominant run for him and the Cavaliers.

Ten wins and no losses to begin the playoffs and the absence of All-Star Isaiah Thomas to possible hip surgery created what’s expected to be an inevitable rematch of Cleveland and Golden State in the NBA Finals. Yet the Boston Celtics are coached sharply and have tenacious players throughout the roster, and they surpassed James on a night his team’s principles went awry and key plays didn’t come from the world’s best player.

As the Warriors aim to sweep the San Antonio Spurs in the West, the Cavaliers squandered a 21-point, third-quarter lead and suffered a 111-108 loss to Boston on Sunday night in Game 3 of the Eastern Conference finals. Cleveland leads the series 2-1, and it could still end sooner than later. But after James posted 11 points, six turnovers and nine missed shots out of 13 attempts, he had an admission.

“I didn’t have it,” James said late Sunday. “I just didn’t. I played poorly.

“But I’m glad it happened. We let our foot off the gas pedal.”

Boston rallied without its emotional leader and All-Star in Thomas, and made this a series – for now. For one night, the Celtics had arguably their best two-way backcourt performance all season from Marcus Smart and Avery Bradley. Smart had 27 points, seven assists, five rebounds and two steals, and continues to cement himself as a cornerstone of this organization, while Bradley added 20 points. The Celtics’ front office celebrated a victory against James, against all odds, and for one night stepped away from preparations for a draft in which it does not yet have a concrete plan for the No. 1 overall pick.

The Celtics faced significant deficits from the tip Sunday night, and they chipped and chipped away. Soon, a 21-point lead turned to 10. Ten turned to seven. Seven turned to two. Soon, the Celtics had gone ahead – and won on Bradley’s 3-pointer that touched every part of the rim before falling in.

Eighteen 3-pointers for the Celtics in all, seven from Smart, and the Cavs’ Kyrie Irving could only roll his eyes.

“Pride, freedom and desperation, that’s what happened,” Irving said. “They played with it all. [Smart] got comfortable, and we let him.”

Cleveland had allowed too much confidence to develop within a beaten-down roster, and perhaps had fallen, too, for the amplifying hype surrounding a possible Cavaliers-Warriors Finals. “That’s made-for-TV commentary that focus can be lost in,” James Jones told The Vertical. “But we don’t pay attention.”

Jones, a three-time champion alongside James, paused. James had been irked throughout the postgame walk to the podium, then to the arena parking lot, but Jones has viewed James this way after previous losses in Miami and Cleveland. It starts with cleansing his mind and demeanor.

“LeBron has been consistent in his bounce-back attitude since Miami,” Jones told The Vertical. “He doesn’t change for us. I know he’s going to come into the facility [Monday] upbeat, like the loss helps us on this process.”

Boston is back in this series, and coach Brad Stevens will have his most potent defensive lineups on the floor now. Thomas does so much for the Celtics – creates shots and energy, confidence and uncanny plays – but Smart and Bradley make up an elite backcourt defensively. The tenor of Sunday night changed when Smart, Bradley and Jae Crowder increased their physicality with Irving and James.

In the end, James accepted his performance with clarity. The Cavaliers had sabotaged their defensive game plans on Sunday night, ending with J.R. Smith losing his assignment for Bradley’s game-winner, and now Boston is within striking distance.

James walked out of Quicken Loans Arena with forceful yet minor interactions, and his pleading was underway for this Eastern Conference finals series to be given the proper respect. A 3-0 series lead was within sight, and these Cavaliers were left searching for answers as the entire basketball world anticipates a third Finals matchup between “super teams.”

“Let’s get back to playing desperate basketball,” James said, “and let all the narrative stop.”

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