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 on Saturday night. "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 seed in the NBA playoffs. They celebrated appropriately: Andre Iguodala, who sank the game-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 from the rafters.

"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 afterwards. "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 they aren't ready to celebrate yet. 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 ever 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 that 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 playoff 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 just 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 he was clearly not moving as well. He said Friday he could run and jump straight, but it bothered him when he tried to move laterally.

He is now wearing braces on both knees, and he said he expects to play on Saturday night.

"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."

Shooting guard 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 on Thursday, shooting 1-for-7 and missing two free throws.

"It puts you somewhat in a disappointing mood, frustrating," said Allen, who planned to stay off his feet on Friday and see how he feels on Saturday. "I'm back to the same place where I was two weeks ago."

Philadelphia won two of three games against Boston in the regular season, both at home, including a 32-point victory on March 7.

Allen said he considered that an advantage.

"Beating them 3-0, if we did, you come into it a little cocky and arrogant," Allen said. "But since they beat us the two out of three, we know this team is capable and they've had our number, so we have got to zone in and focus on beating them and taking care of all those things we didn't do well."