The Boston Celtics entered the Eastern Conference semifinals cruising down the easiest road back to the NBA Finals they may ever see, and for some reason, they seem intent on taking the path of most resistance.

They opened Game 4 with a 2-1 lead and all the urgency of a hungover sloth. The Celtics found enough energy to erase a 16-point, third-quarter deficit and force overtime against a Philadelphia 76ers team trying equally hard to give them the series, but Boston stalled its final possession so much that Marcus Smart's potential winning 3-pointer failed to beat the game clock by a fraction of a second in a 116-115 loss.

It is hard to say which is the bigger failure — the Celtics' lackluster performance through three quarters or their absentmindedness on each team's final play. Either way, they blew a golden opportunity to push the Sixers to the brink. The series is now tied, 2-2. Game 5 in Boston is scheduled for 7:30 p.m. ET Tuesday (TNT).

"We had that game in our grasp," said Boston's Jaylen Brown, who scored 23 points on 10-of-16 shooting.

It would be a mistake not to credit Philadelphia's James Harden, who brushed off consecutive subpar games to score 42 points, including the winning 3-pointer with 19 seconds left in overtime. His 45 points previously took Game 1 from a Celtics team ill-prepared to face the Sixers without MVP Joel Embiid.

Yet, even Sunday's winner was the result of a Boston brain fart. Trailing 115-113 as time waned in the extra frame, Embiid backed Jayson Tatum into the paint, where Brown opted to double-team that game-tying opportunity, leaving the corner open for Harden — "hotter than fish grease" — to win it from distance.

"Big shot by James Harden, but that's my fault," said Brown. "I take full accountability. It was a bad read."

"We were up two with 20-something seconds left, so we wanted to not give up any 3s and not foul," said Tatum. "If they scored a 2, we would be fine, but things like that happen in the course of a game."

The Celtics had nearly a full shot clock to respond, and they did not. First-year head coach Joe Mazzulla opted not to call a timeout, even as Smart still held the ball at halfcourt with nine seconds left and no play developing. Smart passed to Tatum, who waited until inside of five seconds to drive the lane. He attracted a double-team, fired crosscourt to Smart again, and the clock expired before he could release his own winner.

"I waited a second too late," said Tatum, who scored 22 of his 24 points after halftime.

"We just had to play with a little bit more pace there," said Mazzulla, "but that was the play."

It was more evidence of Boston's inability to seize the moment that can break an opponent. The Celtics had been there just five minutes before, when Mazzulla did not call a timeout on the last possession of a tie game in regulation. They ran a similar play — Tatum drawing a second defender, dishing to Smart at the arc — and this time the worst shooting option on the floor for Boston front-rimmed the winning attempt.

"We know what we're supposed to be doing," said Harden. "It's Game 4. They know our plays. We know their plays. It's a matter of who wants it more, who wants to win, and tonight we did a good job of that."

It is fairly apparent the Celtics have the most talented rotation of any left in the playoffs. At their best, they are the NBA's most formidable force on both ends of the court, a relentless eight-man collection of versatile two-way performers. The Celtics have the least flaws, only theirs is most frustrating: They play themselves.

"We let them get comfortable," said Smart.

"Tougher team sets the rules," added Brown, "and tonight that was them."

Last year's blown leads in Game 5 of the conference semis against the Milwaukee Bucks, Game 6 of the East finals against the Miami Heat and Game 4 of the NBA Finals against the Golden State Warriors all would have paved an easier path for the Celtics. Their no-shows for prolonged stretches of Games 1 and 4 of this series, as well as Game 5 against the Atlanta Hawks in the last round, are self-imposed roadblocks.

Embiid's injury and the elimination of the top-seeded Bucks opened an E-ZPass lane to another Finals. Whether it is Embiid's sprained knee or not, it is clear he is not conditioned for four quarters of high-level playoff basketball, despite back-to-back 30-point outings. He is 5-for-19 with 2 assists in three fourth quarters and one overtime since returning from the sprain. (That second assist found Harden on Sunday.)

"We got tired, honestly," said veteran Sixers coach Doc Rivers, "and it's tough to draw up a play when everybody's fatigued. Let me say that. I used one timeout just to give us some rest, and I thought we kind of gathered ourselves, got our wind back. ... We executed our butt off in overtime. That was good to see."

The Heat or New York Knicks await in the conference finals, and more people than not picked both teams to lose in the opening round. A chance to play for the championship — the "unfinished business" that is their playoff slogan — is at arm's length for the Celtics, only they have to flex on Philadelphia first.

Harden has put together a pair of all-world outings, and still Boston had every chance to win both, if not for poor execution (and strategy) on both ends when the pressure cooks. Mess around more, and Harden is bound to steal the Celtics' lunch. They are truly starved if Harden is feasting more than them in the clutch.

The Celtics came so close to securing a 3-1 lead in last year's NBA Finals, they could taste a title. They entered these playoffs knowing they cannot afford to play with their food, only they are still not hungry.