LONDON – We've seen this movie before.

What happened Thursday afternoon at The 02 Arena was a 76ers' re-run. They jumped out to a sizable lead, watched it gradually slip away and lost. This time, it happened in a 114-103 NBA London decision to the Boston Celtics.

The Sixers (19-20) blew a 22-point second-quarter lead against the Celtics en route to having their four-game winning streak snapped. Boston (34-10) took its first lead of the game, 69-68, on Daniel Theis' dunk with six minutes left in third quarter. It didn't look back.  The Celtics went on to lead by as many as 19 points en route to an 11-point win.

In all, the Sixers were outscored 87-54 after holding a 49-27 advantage with 6 minutes, 56 seconds left before intermission.

But Ben Simmons said this defeat, another in a long line of double-digit leads wasted, was different than the others.

"That's the first time we ever let somebody back in and not done anything about them [making a] push," the rookie point guard said. "So I just think they just made a run, and we didn't do anything about it.

"We didn't execute plays. We didn't get rebounds. We didn't get stops. That's what the game came down to."

Why weren't the Sixers able to fight back?

"I have no idea," Simmons said. "It didn't feel the same as any other game. That was the difference. It didn't feel like the normal push that somebody made."

The Sixers were outscored, 37-22, in the decisive third quarter while making just 1 of 6 three-pointers and committing six of their 19 turnovers.

Joel Embiid continued to struggle from the field against the Celtics.  He finished with 15 points and 10 rebounds for his 17th double-double of the season, and added five assists, two steals, one block and four turnovers. However, the center made just 6 of 17 shots, including 1-for-5 on three-pointers.  Against Boston on Oct. 20, he made just 4 of 16 shots. That's 10-for-33 (30.3 percent). Embiid didn't play when the teams met on Nov. 30.

Embiid doesn't think Celtics big men Al Horford and Aron Baynes do anything specific that other defenders have tried  against him.

"I think tonight I started off really frustrated by the way everything was set," he said. "By the way, if I'm not having fun … I know the way for me to have fun and kick someone's [butt] is also talk trash. If I'm not doing that, usually I have bad games.

"I don't think they did anything. I was just not in a rhythm and I didn't do a good job of letting my teammates find me, either. I don't think they did anything special."

JJ Redick (10)  and Simmons (six) combined to score the Sixers' first 16 points as they jumped out to a seven-point lead.  Embiid finished the quarter 0-for-4.

According to Embiid, part of his and the team's struggles had to do with their third-quarter performance.

"In the third quarter, we didn't move the ball," said Embiid, in response to being asked what led to being out of rhythm. "We kept going back down, trying to find a mismatch.  I don't think it was a rhythm. I think I wasn't finding my spots."

Redick led all scorers with 22 points in the losing effort. Simmons added 16 points to go with three assists and three turnovers.  Robert Covington struggled, scoring five points on 2-for-8 shooting (1 of 5 on threes) and had four turnovers.

Jaylen Brown paced the Celtics with 21 points. Kyrie Irving added 20 points and a game-high seven assists, while Marcus Morris had 19 points off the bench.

Morris was involved in a heated exchange with Simmons with 4:48 left to play in the fourth quarter.

Simmons knocked the North Philly native down on the court while leading with his right shoulder fighting through a screen.  Morris got up, shoved Simmons and had to be separated from the Sixers rookie.  Simmons was called for a foul on the play while Morris received a technical for his actions.

"He just took a cheap shot," Morris said. "[Shoot], it's all right, I take a lot of cheap shots, too. It's good to see that Philly got some OK guys, some tougher guys. I'm sure that won't happen, again.  But it's a good shot.

"Hopefully, I don't have to get fined for that."

Said Simmons: "It was a strong hit. I guess he didn't like it. That's about it."

But the end result was all too familiar for the Sixers.

They blew an 18-point, third-quarter lead en route to losing to the Portland Trail Blazers on Dec. 28. Seven days before, they surrendered a 22-point, third-quarter advantage in a loss to the Toronto Raptors. And they blew a 24-point halftime lead before falling to the Golden State Warriors on Nov. 18.

In all, the Sixers have lost seven games in which they led by 11 points or more. The Celtics, meanwhile, extended their winning streak to seven games.