Kevin Garnett soars for a dunk early in the first quarter of Wednesday night’s Game Five against the Hawks in TD Banknorth Garden.

The Providence Journal / Mary Murphy

BOSTON –– At some point in this entertaining, if not strangely odd, first-round playoff series, the Boston Celtics have a few questions to ask themselves.

Are the Celtics as dominant as their 66 regular-season wins indicate? Were they simply caught off-guard by the youthful, energetic Atlanta Hawks last weekend, and was order restored in a 25-point Game Five beat-down?

Maybe the Celtics tore through the regular season while on a mission, more focused than the flotsam and jetsam of Bobcats, Knicks and Heat scattered around the NBA’s Eastern Conference. Now that it’s playoff time and every team is playing for its life, do the Celtics really merit a powerhouse label?

Answers to some of the above will come from the rafters of a sold-out Philips Arena tonight at 8 in Game Six of a series that Boston leads by 3-2. The Celtics can knock out the pesky Hawks with a win. Or the underdogs can keep nipping at the top-seeded team’s heels and tie the series with a third consecutive home win.

“We just have to go into Atlanta and get it done. It’s the biggest game of the year for us,” said guard Rajon Rondo. “We have to go in and focus and get this Game Six.”

The Celtics and their fans could very well see a ballgame at the Garden on Sunday. If the Celts and Cleveland Cavaliers (who play at Washington) both win tonight, LeBron James and his teammates come to Boston for the start of the next round of playoffs.

But the Celtics could also suit up Sunday for a Game Seven with the Hawks. No one –– not the coaches, players or even the fans –– think that’s a very good idea.

“I’ve never been in a playoff series, but I don’t think I want to go to a Game Seven. You want to avoid that as much as possible,” said valued reserve Leon Powe.

To get rid of the Hawks and welcome either the Cavs or Wizards on to their scouting reports, the Celtics need to carry as much intensity as they can to Atlanta and match the energy the Hawks seem to play with only at home. The Hawks averaged 99.5 points in two home wins and 81 points in three trips to Boston. That’s a head-scratching 18.5-point swing.

“The bottom line is we’re not going to beat the Celtics playing half-court basketball," said the Hawks’ Josh Smith. "We’re young and we’re athletic and we have to use that to our advantage. Their advantage on defense is for us to walk the ball across the half-court line and run half-court sets. And we’re just not going to beat this team doing that.”

The ultra-athletic Smith is on to something. When he’s allowed to run the floor and take passes from a speeding Mike Bibby and make plays going to the rim, he makes Celtics such as Kevin Garnett look old. When Garnett and the rest of the Celtics’ defense digs in, Smith becomes a jump shooter. The only Hawks’ shooter the Celtics truly worry about is the dangerous Joe Johnson.

In Game Five, Johnson picked up two fouls by the 4:26 mark of the first quarter and had to leave the game. By the time he returned in the second quarter, the Hawks were trailing (41-28), and the Celtics’ defense was rolling out smoothly. The Hawks were down by 15 at halftime but made a strong run that sliced their deficit to 60-54 midway through the third, but Boston answered with a Garnett post move and two 3-pointers from Ray Allen highlighting a 12-2 knockout punch.

Are the Celtics destined to be pushed to the limit by a team that owns a 12-32 road record?

“We have to do it again, we have to do it on the road now," coach Doc Rivers said. "It’s going to be an amazing atmosphere down there and we’re going to have to go in there and play like (Game Five). If the offense wasn’t as good as it was tonight, then we still have to win the game with our defense. The thing I liked was both were good tonight. But at some point, we’re going to have to win a game when our offense isn’t as good and our defense has to be great.”

kmcnamar@projo.com