ATLANTA — The Hawks were back home Thursday, feeling a lot more comfortable as they watched film and practiced for nearly two hours at Philips Arena.

“We’re very confident,” said guard Joe Johnson, who is averaging 21.8 points in the series. “We’re 2-0 at home. We’ve been playing some pretty good basketball here. We’ve just got to continue that.”

But those hopeful words come with a mighty big caveat: Even if Atlanta manages to beat the heavily-favored Celtics in Game 6 Friday night, the series would be decided in Boston.

And the Hawks haven’t come close to a Beantown upset.

“Boston has killed us three times up there. We’ve won twice here,” Johnson said. “So the home-court advantage definitely plays a big part.”

The Celtics romped to their third straight blowout at home on Wednesday night, winning Game 5 110-85 and reclaiming the upper hand in a best-of-seven series that’s already lasted longer than just about everyone expected.

Top-seeded Boston took care of two prime objectives by stifling Johnson, who got off only 11 shots after a 35-point effort in Game 4, and dominating the Hawks on the boards (39-28).

Atlanta couldn’t find anyone to pick up the slack for its leading scorer, with the other four starters and sixth man Josh Childress combining to hit just 19-of-50 (38 percent). The Celtics, on the other hand, lit up the hoops by making nearly 54 percent of their shots, including 9-of-20 outside the 3-point arc.

“The execution was as crisp and as sound as we’ve had in a month, and the defense was terrific,” Boston coach Doc Rivers said. “We were trapping, smothering and we didn’t make a lot of adjustments. We just did what we should do, and we did it well.”

The Celtics made a point to get Paul Pierce more involved at the offense, and that paid off with 22 points and six assists. Kevin Garnett added 20 points to go with his usual stellar defensive work, which included three blocks. The other member of the Big Three, Ray Allen, kept getting open looks and burned the Hawks with five 3-pointers.

The supporting case also chipped in. Sam Cassell, Rajon Rondo and Leon Powe all scored in double figures. James Posey hit two pivotal 3s.

Clearly, there’s a reason the Celtics are one win away from 70 this season.

“They hit a lot of tough shots,” said Childress, shaking his head. “Some of the stuff was our coverage, but they hit a lot of tough shots. Sometimes, you just can’t do anything about it.”

The Hawks, who went 37-45 during the regular season, turned in two of their best performances the last time these teams were in Atlanta. But one wonders if they’ll be able to muster that sort of passion Friday night, knowing their reward for winning would be another trip to Boston, where they have lost three times by an average of 22 points.

“It doesn’t take any edge away,” Johnson insisted. “We feel like if we have three or four tries up there, we should at least be able to get one. That’s what we’re banking on right now.”

The Celtics would like to finish this one off as soon as possible, having already expended more effort than they might of wanted in an emotional, physical series that’s even turned nasty at times. If they want to avoid Game 7, they’ll have to pack the stifling defense they’ve shown at home.

“We’ve got to carry that same intensity on the road,” Garnett said. “At times, we do it in spurts, but you know when we really lock in and decide that we are going to make some stops, we really lock in.”

The Hawks must find a way to free up Johnson when he’s got two or three defenders running at him every time he gets the ball, which is what the Celtics did for 48 minutes in Game 5 and will surely do again Friday night. If he can’t get open, the other four players on the court must make the extra pass, find the open man and hit the shot.

“It was very frustrating,” Johnson said. “But there’s nothing I can do about it. They’re not going to change.”

Who must change for the Hawks?

Start with Mike Bibby, who probably should have kept his mouth shut when it comes to Boston’s fans. Still remembering that he accused them of being “fair weather” supporters after Game 1, they booed the Hawks point guard every time he touched the ball. He responded with another miserable performance on the road: 2-of-8 shooting and only one assist.

But the rest of the Hawks weren’t much better.

Marvin Williams, who was supposed to be a big part of the offensive plan if the Celtics clamped down on Johnson, had only 12 points. Josh Smith, coming off two huge games in Atlanta, was held to 4-for-13 shooting. Rookie Al Horford was the only one who stood up to the Celtics with another double-double, 14 points and 10 rebounds.

With Boston making more than half its shots and crashing the boards on those that missed, the Hawks had few chances to run — the key to creating offensive chances against one of the league’s best defenses.

“We can’t allow them to dictate our game,” Childress said. “We have to dictate the game. In this series, it all starts at the defensive end.”

It also starts at home, which is why the Celtics weren’t too concerned even after they lost two in Atlanta.

“There’s no panic,” Cassell said. “They just did what they had to do at home, they tied the series up, so we had to take care of our home court. You see, people don’t realize this is why we work so hard during the course of the regular season — to get home-court advantage.”