So, they did it again, winning 92-83, and coming from an immense physical and emotional deficit to tie the series at two games each and ensure, if nothing else, that there will be at least one more game in the Wells Fargo Center this season.
Nothing else is certain, because it is hard to predict the effect this game will have on the Celtics, who thought most of the evening that they had found the exit door leading to the conference championship.
Maybe Boston will regroup on its home court Monday in Game 5, or maybe the Sixers will continue to defy the laws of basketball logic. If they can steal another game in Boston, where they won the second game of the series, then the next game in the Wells Fargo Center might not be the last game for the Sixers, but the last one this season for the Celtics.
Boston was ahead, 49-31, just a minute or so into the third quarter when the Sixers revved their defense into gear and began to turn the game. Suddenly, the Celtics were getting one shot, usually contested, and nothing more. Suddenly, the Celts were turning over the ball, and fouling to stop the Sixers' fastbreak.
"We lost our composure," Boston coach Doc Rivers said. "We stopped running our stuff."
The complete turnaround took until early in the fourth quarter, and then the game, as ugly as ever, became beautiful. Boston woke up and started playing again, and the Sixers never stopped. They got huge baskets from Lou Williams and Andre Iguodala down the stretch and put away the game at the foul line.
"I don't know what clicked in," coach Doug Collins said. "We started moving the ball and we were just going to keep fighting. That's what we do. It ain't pretty, but we keep grinding. Now, we've got a chance for a Game 5, and I told the team that we'll just go and see where we go from here."
If the Sixers hoped to quickly erase the memory of their blowout loss in Game 3 and regain some momentum in this series, the opening quarter couldn't have gone much worse. Collins said the Sixers had to come out with a better defensive effort than they showed on Wednesday, and, if possible, it wasn't even that good at the start of Game 4.
"We know they're going to come out with the idea that if they get the game tonight, they won't have to come back here and they'll get some rest," Collins said. "Teams that have championship aspirations, the whole thing is to play as few games as possible. The more you play, the more chance for injury and you don't rest. There's no question teams think like that."
Boston is a veteran team, with veteran legs at this time of the year, and there's no doubt they would like a breather between series if they are able to advance. Having the killer instinct to follow through on that hasn't been their specialty. They came out quickly in a possible clinch game against Atlanta in their opening-round series and let the Hawks back into it, prolonging that series. Could they do better against the Sixers in a game that would give them control heading home for Game 5?
"I would like that, but we haven't been very good in those," Rivers said before the game. "This is a big game. I tried to tell our guys that we really haven't done anything yet. Hopefully, we can make some room for ourselves tonight."
It appeared that the Celtics were making plenty of room for most of the game, but at the end, they ran out of room and they couldn't run away from the Sixers. Their hope of taking a commanding lead in the series and putting it away with a leisurely home game is gone.
If the Sixers were the team doubting themselves after Game 3, then Boston, a team whose experience should make it immune to such things, has to face the possibility of a bad outcome to the series now.
They couldn't put away a game that was over, and they couldn't take control of a series that was theirs to control. That's because the Celtics are used to operating in the normal world of the NBA, and their opponent rarely visits there.
Contact columnist Bob Ford at email@example.com, read his blog at www.philly.com/postpatterns, and follow on Twitter @bobfordsports.