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!” he 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, have now 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’s making 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 with a sprained left knee. Kevin Garnett had 22 for the Celtics and Ray Allen added 20.

Also on Friday:

Cavaliers 105

Wizards 88

WASHINGTON — LeBron James finished with 27 points, 13 rebounds and 13 assists for his third career playoff triple-double, all the while helping slow Caron Butler at the defensive end, and Cleveland ended its contentious series with Washington in six games.

Wally Szczerbiak added a career playoff-best 26 points and reserve Daniel Gibson had 22, combining for 10 of Cleveland’s 11 three-pointers.

Jazz 113

Rockets 91

SALT LAKE CITY — Deron Williams scored 13 of his 25 points in the decisive third quarter, and Utah won the series 4-2 to knock Houston out of the playoffs in the first round for the second straight year.

Six other players scored in double figures for the Jazz, who open their conference semifinal series versus the Lakers in Los Angeles on Sunday.

The Rockets’ Tracy McGrady had 40 points, 10 rebounds and five assists.