WALTHAM, Mass. – The Philadelphia 76ers danced in celebration after their first-round upset of the top-seeded Chicago Bulls.

The Boston Celtics plodded off the court after clinching their first-round series, in search of the nearest bucket of ice.

“They were excited. We were relieved,” Boston Coach Doc Rivers said Friday, the only off day before the Celtics and Sixers begin the Eastern Conference semifinals tonight. “When the clock went off for us we were like, ‘Oh my goodness. Let’s go to bed.’ “

Philadelphia took advantage of a Chicago team missing Derrick Rose and Joakim Noah to become just the fifth No. 8 seed to knock off a No. 1 in the NBA playoffs. They celebrated appropriately: Andre Iguodala, who sank the winning free throws with 2.2 seconds left, hopped on the scorer’s table as the 76ers’ anthem blared in the arena and confetti fluttered down.

“I don’t even know what to say. I really don’t. What this means to me and the organization, and our team. It’s just off the charts,” Coach Doug Collins said. “Sometimes you just can’t figure this out. Sometimes you’ve just got to enjoy it. That’s what I’m sure going to do, I can tell you that. It’s been an incredible year.”

The Celtics eliminated Atlanta in six games but aren’t ready to celebrate.

Boston has advanced at least to the second round of the NBA playoffs in every year since the new Big 3 was assembled in the summer of 2007, twice reaching the finals and winning it all in 2008.

The Sixers had not won a playoff series since 2003, and only a few Sixers have gone this far in the postseason.

“I told them, ‘Now we’re in the high-rent district. Now you start really finding out what it’s all about,”‘ Collins said. “(You) can’t put a price on experience, what our guys are gaining every single day. Just to watch the joy they had in that locker room after the game was something I’ll never forget.”

Rivers said he didn’t expect the 76ers to be a pushover because they were happy just to finally win a series. Sometimes the excitement of that first win can carry a team in later rounds, he said.

“They have the emotional advantage because of their win,” Rivers said. “We have to match that.”

Rivers has more than emotion to worry about.

Paul Pierce has been nursing a sprained medial collateral ligament in his left knee and though he had 18 points in Game 6, was clearly not moving as well. He said Friday he could run and jump straight, but it bothered him to move laterally.

He has braces on both knees but expects to play tonight.

“It’s probably not going to be better until the season’s over,” Pierce said. “I just can’t overdo it, do something that could hurt my career.”

Ray Allen also reported a setback in his recovery from bone spurs in his right ankle. Allen missed about three weeks, including the first two games of the playoffs, then returned for three solid games before struggling Thursday, shooting 1 of 7 and missing two free throws.

THE NBA said a foul against Boston with 3.1 seconds left Thursday night should have been called sooner, which would have given Atlanta a free throw instead of just the ball out of bounds.

Marquis Daniels of the Celtics held Al Horford as Atlanta was inbounding the ball trailing 81-79.

Referee Eric Lewis called a foul, but ruled it came after Marvin Williams released the ball, meaning it was just a common foul that resulted in another throw-in.

But replays showed the foul occurred before the ball was passed and should have been treated as an away-from-the-play foul. Atlanta would have been awarded a free throw and retained possession.