Boston guard Sam Cassell (28) is restrained by teammate Tony Allen (42) while arguing with a referee. Atlanta stunned Boston again, 103-100, Friday in Atlanta.

Atlanta ? As streamers poured from the rafters at Philips Arena, Marvin Williams stood at center court with arms folded, looking defiant, even with a sore knee. Zaza Pachulia grabbed a microphone and summed up this improbable series.

“We’re going to Game 7! Woooooooooo!” Pachulia screamed.

Game 7, indeed.

Who would have believed it?

Refusing to let go of their first trip to the playoffs in nine years, the Hawks forced an improbable deciding game with the Boston Celtics when Joe Johnson hit a crucial three-pointer with 1:07 remaining, and Atlanta held on for a 103-100 victory Friday night.

The Hawks, just 37-45 during the regular season, now have beaten the Celtics three times in the space of a week to set up a deciding game in a series that was supposed to be a sweep. After all, Boston went an NBA-leading 66-16 and defeated Atlanta in all three meetings before the playoffs.

But, against all odds, the Hawks are heading to Boston for Game 7 on Sunday.

“Seven! Seven! Seven!” chanted the record crowd of 20,425 at Philips Arena.

The Celtics are still the obvious favorite, having won all three games at their place by an average of 22 points. But the Hawks have lasted longer than anyone would have expected.

“I feel very good,” said Pachulia, a little-used center who has made quite an impact in this series. “You’ve got to be able to win on the road. We’ve lost three in a row. We should be able to win one up there. It would be sad if we lost four in a row.”

The Celtics looked as shell-shocked as anyone. Paul Pierce fouled out and spent the final minutes on the bench with a towel draped over his head, barely able to watch.

“It’s definitely a dream,” Atlanta’s Josh Smith said. “But we weren’t going to be satisfied with just making the playoffs.”

Williams led the Hawks with 18 points despite missing much of the fourth quarter due to a sprained left knee. Kevin Garnett had 22 for the Celtics, and Ray Allen added 20, but Pierce – the other member of Boston’s Big Three – fouled out on a disputed call with 4:44 remaining.

The Hawks kept the ball from Allen and James Posey on the final possession, forcing Rajon Rondo to launch a long three that didn’t even hit the rim as the horn sounded.

For the third time in the series, streamers poured from the Philips Arena rafters.

“I’ve played 13 years, and I’ve been here every year for 13 years,” Garnett said. “This is the first time I’ve seen this atmosphere like this (in Atlanta). That’s the intensity of the playoffs.”