BOSTON -- As one second after another ticked off the clock, Philadelphia 76ers star James Harden continued to eye both the basket and his defender, Boston Celtics center Al Horford, with the Sixers trailing by one in the closing seconds of Game 1 of the Eastern Conference semifinals Monday night.

And, like he has countless times before in his Hall of Fame career, Harden saw the moment when he could create the separation necessary to get off his patented step-back jumper -- and took it.

When the ball softly dropped through the net for the final time for Harden on the night -- tying his playoff career high of 45 points in the process -- teammate Joel Embiid celebrated wildly on the sidelines, Harden and the 76ers having stolen back home court with a thrilling 119-115 victory without him.

"I haven't felt one of those zones in a minute," Harden said with a smile afterward. "You know what I mean? Just to be aggressive and shoot the basketball and do what I want.

"That felt really good. ... I'm capable of doing it, so it felt good. It felt good to make those shots, to give ourselves a chance."

Few believed Philadelphia would have much of a chance once Embiid, dealing with a sprained LCL in his right knee suffered April 20, was officially ruled out roughly 90 minutes before Game 1 tipped off.

But few also would have believed that Harden was going to turn in perhaps the greatest performance of his playoff career. He came out and hit his opening five shots, setting the tone for Philadelphia on a night when the game threatened to get away from the 76ers early thanks to Boston going 17-for-20 from the field in the first quarter alone.

That the game was still within striking distance was almost solely due to Harden.

"I thought that he had just the perfect mindset tonight," 76ers coach Doc Rivers said. "He really did. I'm so happy for him because it just tells you what he can do on given nights.

"The guy is a Hall of Famer, and all you hear is the other stuff about him. And he was fantastic."

With Embiid sidelined, Philadelphia put an emphasis on doing something the Celtics often do: trying to win the math game by outshooting Boston from deep. Harden -- who finished the game 17-for-30 from the field, including 7-for-14 from 3-point range -- shot more than half as many 3-point attempts as the entire Celtics team (26) and made nearly as many (Boston finished with 10 triples).

And none of them was bigger than the one he hit over Horford, which came after a wild sequence that saw Malcolm Brogdon throw a pass straight to Tyrese Maxey with 0.1 seconds left on the shot clock, allowing Maxey to race ahead of the field and lay the ball in to give Philadelphia a one-point lead.

When Jayson Tatum was fouled on the ensuing offensive possession, Rivers subbed in Georges Niang for Paul Reed before Tatum hit his second free throw, electing not to use his final timeout and instead presenting Harden with a chance to hunt out his preferred matchup.

And, once Harden realized the Celtics weren't going to send a double-team at him, he did exactly that.

"I came off the screen and I was wondering if they were going to put two on the ball," Harden said. "And so when I cross-screened and pulled the ball back out, it was just like, 'Stay home, it's a one-on-one.' ... Then I'm just looking up and I'm just [saying], 'All right, this is what I work on every day.'

"Get the best available shot no matter what it is. And, you know ... raise up and shoot it."

Afterward, Celtics coach Joe Mazzulla was asked whether he thought Boston should have tried to double-team Harden on that final possession, rather than leaving him isolated against Horford, given the rhythm he got into during the game.

"We doubled in the first half, and they got six points off double-teams on Harden," Mazzulla said. "Hindsight is always 20-20. If we didn't double him, it would have been great defense. If we did double him, and they hit a shot, it would've been, 'Why did we double him?'"

But the Celtics didn't double Harden, and he rose up and buried a dagger into Boston and its sellout crowd of 19,156, giving the 76ers a monumental victory.

Not only did the 76ers steal back home-court advantage in the series, but they did so without Embiid -- who could become the NBA's Most Valuable Player on Tuesday night, when either he, Denver Nuggets center Nikola Jokic or Milwaukee Bucks forward Giannis Antetokounmpo will be named the winner.

Now, as Embiid recovers from that sprained LCL, the 76ers have already accomplished what they set out to do upon arriving in Boston.

"I think we take pride in playing without the big fella," 76ers forward P.J. Tucker said. "As good as he is, I think it gives others opportunities to step up and play. James can kind of do his thing, Tyrese is obviously really good without Jo, and everybody else ... everybody took a little step up with him out. It's like a pride thing for us."

It was clearly a pride thing for Harden, who said after Philadelphia's Game 4 win over the Brooklyn Nets to close out a first-round sweep that his focus this season has been on sacrificing for the betterment of the team. But, on this night, with Embiid sidelined, it wasn't about sacrificing -- it was about being aggressive and trying to carry Philadelphia to a win.

"I don't need to make a statement," Harden said. "My coaches, my teammates, what they expect me to do all throughout the course of the year was be a facilitator and get Joel the basketball and score when necessary. Joel wasn't here tonight, you know what I mean? And, we knew that going into this series.

"Now it's like, 'All right, open the floor. James, you be aggressive.' And tonight, I was aggressive. So, it's not that I'm not capable of doing it, it just ... this is my role for this team. Now, if you want me to do [what I did] tonight, then I can do that as well.

"I don't think a lot of players can do that. So yeah, I appreciate that."