ATLANTA — Four years ago, Boston and Atlanta were in much different places when they met in the NBA playoffs.

The Celtics, a champion in the making. The Hawks, a team on the rise.

This time, they’re in a similar situation: playing to stay together.

The Celtics are possibly making a last stand with their aging Big Three — Kevin Garnett, Paul Pierce and Ray Allen. The Hawks have a younger core but desperately need a long run in the playoffs to justify keeping all the important pieces intact, especially dynamic forward Josh Smith, who has one more year on his contract and a checkered relationship with his hometown team.

Game 1 in the best-of-seven series is Sunday night in Atlanta, which has home-court advantage despite being the lower-seeded team.

“Both teams are playing for their survival as a group,” Celtics coach Doc Rivers said Saturday. “If we win the series, and keep winning, you’re going to want to build on this group. If we win, you may not see that (Hawks) team anymore.”

Atlanta has made three straight trips to the second round, including a tough six-game loss last year to the top-seeded Chicago Bulls. But the Hawks have never won more than one playoff series in any given year since moving from St. Louis in 1968, which has turned the city into something of a basketball wasteland.

When it comes to attendance, Atlanta generally ranks near the bottom of the league no matter its record (this year, the Hawks were 23rd overall and second-worst among playoff teams). Many of the fans who do turn out in this city of transplants tend to root for the other team, which was certainly the case for Boston’s two visits to Philips Arena this season.

Atlanta star Joe Johnson is hoping the fans will be more like there were in the 2008 playoffs, when the Hawks won all three at home before raucous, supportive crowds to surprisingly force a Game 7 against the top-seeded Celtics.

“There was so much excitement here in Atlanta,” Johnson remembered. “Just the way the crowd was into it and made this a very hostile environment. Man, it was pleasing to play in this building. It was a lot of fun.”

What does he expect this time?

“I know it’s going to be loud,” Johnson said, breaking into a sly grin. “I just hope it’s going to be in our favor.”

Neither team is at full strength. Allen has not played in two weeks because of a sore right ankle, and he’s not sure if he’ll be ready to go in the series opener. In fact, if this wasn’t the playoffs, the 36-year-old shooting guard would already be having surgery. Instead, he took a cortisone shot a few days ago and said Saturday the ankle “feels a lot better.”

“As I stand here, just moving around on it, walking around, it feels great,” Allen said at his team’s suburban Boston practice facility, before the team flew to Atlanta. “Now, transferring it out on the floor and moving around on it is the next step.”

The Celtics won’t get any sympathy from their opponent. Atlanta already played most of the season without center Al Horford, who had hoped to be recovered from pectoral surgery in time for the playoffs but has already been ruled out for the Celtics series. The guy who took his starting spot, rugged Zaza Pachulia, missed the last seven games of the regular season with a sprained left foot.

While coach Larry Drew has tried to be coy about Pachulia’s status for the playoffs, saying again Saturday that it will be a game-time decision, the players have already accepted having to start the postseason without their top two centers.

“With us not having Al and Zaza, it’s going to be tough,” Johnson said.

If Pachulia is indeed out, Drew will have to decide on which way to go with his lineup. He could stick with third-stringer Jason Collins as the starting center and leave everyone else in the roles they’ve played so well, including Smith at power forward and Marvin Williams coming off the bench with a highly effective second unit. Or, the Hawks could go smaller and quicker with their first five, moving Smith to center and promoting Williams to the starting lineup.

No matter who’s on the court, Smith figures to spend plenty of time tussling with Garnett at both ends of the court. That figures to be one of the key matchups in the series, along with the one at point guard, where Atlanta’s Jeff Teague gets the difficult task of trying to contain Rajon Rondo. Boston’s superb floor leader averaged more than 14 assists a game and dished out 20 in an overtime win over the Hawks on April 11.

The teams are much more closely matched than they were before Boston’s playoff victory in 2008 — the Hawks finishing one game ahead in the East standings, the Celtics winning two out of three meetings, none of which was decided by more than five points.

Another seven-game marathon is not out of the question.

“They’ve got three Hall of Famers on that team,” Johnson said. “They’re a great veteran group that knows how to get it done.”

He feels good about his own team, too.

“We’re a lot better, a lot deeper, than we’ve ever been. I think now’s the time,” Johnson said. “The main thing I want to do is try to bring a championship here to Atlanta. The city is well overdue. Why not give it a try?”