The Hawks opened the playoffs with an electric display of running, shooting and scoring that suggested they finally had figured out how to break free from Boston’s suffocating defense.

Alas, it hasn’t been the same since for the Hawks.

“In the first two, three quarters we are probably at our best running and getting up down the floor,” Hawks guard Joe Johnson said. “Then in that fourth, for whatever reason — fatigue maybe — we just get a little stagnant, and our offense just doesn’t flow as much.”

That’s the main reason the Hawks head to Boston for Game 3 on Friday with the best-of-seven series tied 1-1 and needing a victory to regain home-court advantage. The Hawks squandered an 11-point lead late in the third quarter of the 87-80 loss in Game 2 as their empty and ugly possessions piled up.

If the Hawks tire late in games, the Celtics seem to regenerate. Hawks coach Larry Drew told his players the Celtics “smell blood in the water” when they sense the Hawks becoming tentative.

“We allowed the pressure to take us out of what we wanted to do,” Drew said. “You can’t allow that to happen, not in the playoffs. You’ve got to do things harder, do things with pace. We were too lethargic, too methodical.”

The Celtics haven’t been much better than the Hawks on offense, but they’ve been stingier when it counts most.

The Hawks scored 31 points in the first quarter of Game 1, but managed only 52 over the next three periods, including 21 in the final 16 minutes of the game. The Hawks scored 44 points before halftime in Game 2 before sputtering to 36 points in the second half, including 20 over the final 16 minutes.

The Hawks may have to score in Game 3 without forward Josh Smith, who leads the team with a total of 38 points in the series. He suffered a sprained left knee and strained his patella tendon late in Game 2 and is doubtful to play Friday. Smith said he planned to test the knee at the team’s light practice Friday morning. He said the knee had improved and he has a “high threshold for pain,” but doesn’t want to push it too far.

“I understand how important this series is, but it’s more than one game,” he said. “I don’t want to go out there too early and risk getting injured more than I have.”

The Hawks won’t soon get back center Zaza Pachulia for reinforcement. He said he suffered a bone chip in his left foot when he sprained his left ankle April 13 and is at least a week away from being cleared to play.

“When I walk it’s fine, but as soon as I put pressure on it, it hurts,” Pachulia said.

The Hawks also are without center Al Horford (pectoral), who is among their more efficient scorers when healthy. They’ve started Jason Collins at center both games, and he’s been effective on defense, but Boston defenders have helped away from him.

The injuries have chipped away at the Hawks’ depth, and their reserves have not filled the void, especially on offense.

Hawks scoring droughts have generally coincided with the times Drew uses a lineup that doesn’t include Johnson, Smith or point guard Jeff Teague. The Hawks have been outscored 19-7 during the 17 possessions those three players have been on the bench at the same time, according to basketballvalue.com

Boston’s bench scoring hasn’t been much better, but coach Doc Rivers has compensated by always having either Paul Pierce or Kevin Garnett on the court. Before Rajon Rondo was ejected near the end of Game 1 and suspended for Game 2, Rivers always had two of Pierce, Garnett and Rondo on the floor.

“I do want to ride my starters, but I don’t want to wear them out,” Drew said. “It’s going to be a long series. ... I have all the confidence in the world in my bench. They’ve been good for us all season, and I’m not going to give up on them now.”