PHILADELPHIA — At the end of one of the stranger games of his career, Jayson Tatum of the Boston Celtics pounded the ball against the court as the final seconds elapsed. The sound of those hard dribbles — each a percussive thud — seemed to fill Wells Fargo Center as thousands of 76ers fans tried to make sense of what had just happened in Game 6 of the Eastern Conference semifinals.
How was any of it possible? How had the 76ers blown an opportunity to secure their first trip to the conference finals since 2001? How had the Celtics seized on such a simple change — sliding Robert Williams into their starting lineup — to boost their defense? And how had Tatum, after having spent most of his evening chucking up wayward jump shots, ultimately preserved his team’s season?
“For 43 minutes, I had to hear them tell me how bad I was,” Tatum said of the fans. “So it kind of felt good to see everybody getting out of their seats, leaving early.”
A strange series full of strange games will go the distance — because why not? — after the Celtics put the clamps on the 76ers in a 95-86 victory on Thursday, forcing a Game 7 on Sunday in Boston.
Both teams are built to win now. These are not young, overachieving franchises. The 76ers are desperate to fulfill the long-awaited promise of their team-building blueprint known as the Process, with Joel Embiid, who recently collected his first N.B.A. Most Valuable Player Award, operating as their focal point. The Celtics, meanwhile, have been using the slogan “unfinished business,” a nod to how close they came to winning it all last season when they lost to the Golden State Warriors in the N.B.A. finals.
An early exit for either the 76ers or the Celtics — and getting bounced from the playoffs in the conference semifinals would qualify — could lead to a summer of change. A win, though, would be seismic.
“Honestly, I wouldn’t want to go to Game 7 in Boston with any other group,” 76ers Coach Doc Rivers said. “I know we’re going to rally. We’ve rallied all year long on the road.”
On Thursday, Tatum rallied from his own struggles. He missed 13 of his first 14 field-goal attempts, a stretch of futility that extended into the fourth quarter. His teammates, he said, continued to feed him positive reinforcement. Keep rebounding. Keep defending. Keep passing. Keep shooting.
Joe Mazzulla, the Celtics’ first-year coach, went one step further.
“I love you,” Mazzulla recalled telling him. “That’s a pretty powerful statement.”
Tatum’s first 3-pointer of the game gave the Celtics an 84-83 lead. He sank another one 39 seconds later. He made four 3-pointers in the game’s final 4:14, turning the arena into a mausoleum. He finished with 19 points, 9 rebounds and 6 assists.
“We rely on him,” the Celtics’ Malcolm Brogdon said. “He’s our guy. And he’s proven that he’s reliable in those moments. I don’t think there’s any doubt in anybody’s minds. It doesn’t matter how many shots he missed in the first three quarters. He’s going to finish the game for us.”
Tatum, a first-team all-N.B.A. selection for the second straight season, has no shortage of confidence. In a walk-off interview with ESPN after Thursday’s game, he referred to himself as “humbly, one of the best basketball players in the world.” It was quite a statement after he shot 5 of 21 from the field.
“I think that shows character that you call tell yourself that when you’ve only hit one shot,” he said later, “and things aren’t going your way, and you’ve got to be the same person with the same morals, the same character whether you’re up or down. And I kept telling myself that. I believe in myself.”
Accordingly, Tatum gave Mazzulla a reprieve — for at least a couple of days. Mazzulla, who was an assistant under Ime Udoka last season, took over as the team’s interim coach a few days before the start of training camp when the Celtics suspended Udoka for unspecified “violations of team policies.” The Celtics removed Mazzulla’s interim tag in February and signed him to a contract extension.
But the pressure on Mazzulla, 34, has only mounted in the playoffs — and during this series, in particular. There was Game 1, which the Celtics lost even though Embiid was sidelined with a sprained knee. There was Game 4, which the Celtics lost in overtime after they forced up a poor shot in the closing seconds. (Mazzulla later apologized to his players for neglecting to use one of his remaining timeouts.) And there was Game 5, which the Celtics lost thanks to a listless display of basketball that had their home fans booing them.
Before Game 6, Mazzulla made a long-overdue change by starting Williams, a defense-minded center, in place of Derrick White — a move that Marcus Smart, the team’s starting point guard, endorsed. In addition to blocking two shots and affecting countless others, Williams had 10 points and 9 rebounds.
“Joe’s learning, just like all of us,” said Smart, who finished with 22 points, 7 rebounds and 7 assists. “I know he’s been killed a lot, rightfully so. He needed to make some adjustments, and he did that, and that’s all you can ask for — for him to just continue to be the best that he can be.”
Tatum described how he and Mazzulla had leaned on each other throughout the season.
“I know there’s a lot of questions and doubts,” Tatum said, “and I’ve told I’m a lot of times: ‘I’ve got you, I’ve got your back. We’re in this together.’”