A pivotal Game 5 awaits the Sixers. Here are five story lines to watch.

BOSTON — Sunday’s overtime victory over the Celtics in Game 4 of the Eastern Conference semifinals was a thriller for the 76ers. But they don’t have time to bask in any afterglow.

This series’ every-other-day cadence means the Sixers have already returned to Boston for Tuesday’s Game 5 at TD Garden. And though they are guaranteed at least two more matchups in a suddenly interesting series — after they nearly squandered a 16-point lead before James Harden’s shots sent Game 4 to overtime and to a Sixers victory — history indicates that Game 5 is especially crucial. When a seven-game NBA playoff series is tied at two games apiece, the winner of Game 5 goes on to win more than 80% of the time.

Needless to say, the Sixers’ season could hit a pivotal point Tuesday night. Here are five story lines to watch.

» READ MORE: Doc Rivers stresses hard, smart play as Sixers approach Game 5 with Celtics

Game 4 demonstrated that Harden can muster a massive performance on less than a week of rest. But so far in this series, the Sixers have gone the way of their future Hall of Fame point guard.

The most cynical assessment would say the Sixers are two Harden game-winning three-pointers away from getting swept. But he was fabulous in Games 1 and 4, posting his two highest-scoring outputs as a Sixer (45 and 42 points, respectively) on efficient shooting throughout each game before hitting those daggers. Harden was also horrific in Games 2 and 3, making only five of his 28 attempts in some stretches of sheer ineffectiveness (particularly while trying to finish at the rim) and others of baffling hesitancy with the ball in his hands.

» READ MORE: Sixers vs. Celtics prediction: Bet on some James Harden regression in Game 5

Something in between those two extremes would likely suffice for the Sixers. As in, being a superb playmaker after leading the NBA in assists during the regular season, and a reliable scorer who can complement NBA Most Valuable Player Joel Embiid and occasionally take over games.

“I don’t know if we need him to be an elite scorer every night,” Sixers coach Doc Rivers said before Harden’s Game 4 outburst. “… We just need him to be elite. And that encompasses, for us, everything: scoring, passing, attacking. He’s done that most of the year for us.”

It’s also worth examining how Harden unlocked himself in Game 4 from a Boston perspective. Jaylen Brown had frustrated Harden with defensive pressure and length in the series’ previous two games. But two early fouls Sunday backed Brown off of Harden, helping the Sixers guard get cooking in the second quarter. Both teams also deployed strategies to get Brown switched off — or onto — Harden as actions developed. Then, Brown took accountability after his gaffe on the final possession of overtime, drifting over to double-team Embiid and leaving Harden open for the corner three-pointer.

Embiid has looked relatively good physically since returning with a sprained knee that would normally keep a player sidelined for between four and six weeks. Yet he called Sunday’s personal performance “terrible,” even after totaling 34 points, 13 rebounds, and four assists.

He went 11-for-26 from the floor, and acknowledged he had “no lift” down the stretch in the fourth quarter. That’s when former teammate Al Horford blocked him three times in the final five minutes of regulation, twice on inside attempts and once on a jumper.

That fatigue was surely a product of playing two games in approximately 36 hours, the quickest turnaround of this series whether it ends in six or seven games. The rest, recovery and ramp-up time between Sunday evening and Tuesday night will be critical for Embiid.

Ditto for his teammates after Rivers shortened the Sixers’ rotation Sunday. Four out of five starters played at least 45 minutes. Jalen McDaniels was the odd man out in Game 4, but perhaps could be in line for minutes again in Game 5.

Rivers semi-mocked a reporter who asked during Sunday’s pregame news conference about his team’s sense of urgency entering Game 4.

“Do I really need to answer that question?” Rivers said. “I mean, you worked on that for 48 hours, and that’s what you came up with? Whatever ‘high’ is, I’m going to assume it’s high.”

» READ MORE: The Sixers were out of gas, out of luck, out of time. Now, they’re alive.

The Sixers made good on that assumption, with hustle plays and intangibles that Rivers had harped on following his team’s losses in this series. More Sixers hit the floor to battle for loose balls, or elevated to tap out rebounds, or lunged into passing lanes. Four Sixers collected multiple offensive boards, while five had at least one steal (four by Harden). The Celtics, by comparison, totaled just three steals as a team.

Perhaps the most impactful example of that playing style: P.J. Tucker’s offensive rebound and underneath putback through contact and his ensuing free throw, which tied the score at 105 with 1 minute, 6 seconds to play in regulation.

Compare that to the Celtics’ Game 3 victory, when they outrebounded the Sixers, 13-6, in the fourth quarter (with four offensive boards) and picked up nine steals in the game to pace their 12-point victory.

Boston continues to be a rough matchup for the Sixers’ dynamic third-year guard. After making just six of his 17 shots Sunday, Maxey’s shooting percentages in this series dipped to 36.6% from the floor and 28.6% from three-point land. That’s a stunning decrease from his regular-season marks of 48.1% from the floor and a career-best 43.4% from long range.

Rivers continues to stress that the Sixers need to get the ball in Maxey’s hands early and often. Embiid will encourage his teammate to keep shooting. Following Saturday’s film session, player development coaches held up a pole with padding during on-court work to simulate the Celtics’ length, while Maxey tried to regain his touch on jumpers and finishes at the basket.

To Maxey’s credit, he remained engaged Sunday by grabbing eight rebounds (five offensive, often while following his own miss) and digging in defensively as best he could, even against All-Star Jayson Tatum.

But figuring out how to be effective against the Celtics is also important for Maxey’s overall career arc. He is likely to see this version of Boston in the playoffs for years to come.

» READ MORE: James Harden delivers gospel truth and Game 4 win

The Sixers nearly blew a 16-point second-half lead Sunday. But that they even had such a cushion was a positive step, Rivers said.

They trailed at the half in each of the series’ first three games. Heading into Game 4, Rivers successfully challenged his team to play from ahead. And although it’s traditionally more challenging to build double-digit advantages on the road, replicating Sunday’s tenacious first-half defense is a place to start. Boston shot 38.3% before the break in Game 4, and committed four of its 10 turnovers in the opening quarter.