PHILADELPHIA -- As Celtics star Jayson Tatum went through one of the worst shooting slumps of his career through the first three quarters of Game 6 of the Eastern Conference semifinals, he continued to get reassuring messages from up and down the Boston sideline.

"Keep going," Celtics costar Jaylen Brown said. "They can't guard you."

"Let him know that he is that guy," Celtics center Al Horford said. "And he's special."

"I love you," coach Joe Mazzulla said.

Yet, Tatum admitted to feeling frustrated in the midst of a 1-of-13 slump at the start of the fourth quarter, unable to hit shots with the Celtics on the brink of elimination Thursday against the Philadelphia 76ers. But he kept looking up at the game clock, reminding himself that he still had time to make a difference.

Tatum finally broke through in the fourth quarter, knocking down four of his eight shots for 16 points, outscoring the Sixers by himself in the final period to help Boston seal a 95-86 victory and force a Game 7. He hit three 3-pointers during clutch time to put the game away, while the Sixers shot 0-for-6 in the clutch.

Tatum became the fifth player in the past 20 years to outscore another team in the fourth quarter of an elimination game, according to research by ESPN Stats & Information. However, no player during that span had three or fewer points through three quarters and then proceeded to outscore the opposing team in the fourth quarter, according to StatsPerform.

"In transparency, s--- was frustrating," Tatum said after the game. "You want to win so bad. You want to play so well. And shots aren't falling, things just aren't necessarily going your way. And you want it so bad."

The Celtics leaned on Tatum to get them out of this situation during last season's conference semifinals series against the Milwaukee Bucks, and Tatum responded with a 46-point masterpiece to force a Game 7 back in Boston. So, the Celtics had confidence leaning on Tatum on Thursday even though his shot was not falling nearly the same way it had in Game 6 a year ago.

"His poise got him going," Mazzulla said. "The standard that is set for him to where if he's not scoring, he's not playing good basketball is wrong. He's had multiple games this series when he's at his best and he doesn't need to score. His communication with his teammates, I thought his defensive intensity, and I thought even his force driving the ball kept him in the game. And so, he plays with a level of poise regardless of how the game is going."

And despite the shooting slump, Tatum continued to maintain confidence in himself. He attempted to leave an imprint on the game in other ways -- he also collected nine rebounds, dished out six assists with two steals and two blocks -- until his shot began falling late in the game.

"Things not going your way and you got to be the same person," he said. "Have the same morals, the same character when things go up and down. I just kept telling myself that I believe in myself 'til it turned around."

Meanwhile, the Celtics stayed in the game despite Tatum's struggles because of their intensity on defense. Boston held Philly to 86 points, its fewest in a game this postseason and second fewest in any game this season. Robert Williams III was inserted into the starting lineup, and both he and Al Horford paid huge dividends for Boston defensively, despite playing just seven minutes together in the first five games.

When Williams or Horford contested a shot, the Sixers shot 3-for-18 from the field. They combined to hold Joel Embiid, who finished with 26 points overall, to 7-for-16 shooting when they guarded him.

"I was ecstatic about it," Celtics guard Marcus Smart said about Williams being inserted into the starting lineup. "To have Rob in there, he changes the game a lot ... that just goes to show you, Joe [Mazzulla] is learning. Just like all of us. I know he's been killed a lot. Rightfully so. He needs to make some adjustments, and he did that. And that's all you can ask for, just continue to be the best he can be. It takes everybody; it's a full team effort."

Now the Celtics will return to Boston for Game 7 on Sunday. It's the eighth time Boston and Philadelphia have faced off in a Game 7, the most between two franchises in NBA history.

And Tatum will be looking to put this game behind him and get off to a much faster start.

"Hopefully, that's a one-time thing," Tatum said. "Hopefully, I start off better, things like that. But if this is what it takes for us to win, I'll go for whatever. Sunday is obviously a big game, Game 7, and I'm excited to go back and kind of like redeem ourselves in front of our home crowd."