Part of what the Celtics are experiencing right now is disbelief – not in themselves, certainly, but in the hairpin way the Eastern Conference first-round playoff series against the Chicago Bulls has turned.

Heading into Game 5 Wednesday night at the Garden, the home team has yet to win in this series. Not even the Celtics’ record-breaking series against the Bulls in 2009, when league marks for overtime games (four) and overtime periods (seven) were set, went in this direction.

“Very, very strange. I honestly don’t recall ever seeing this happen,” Al Horford said.

Said Jae Crowder: “Never saw a series like that before. We were just taking it one game at a time.”

That dreadful, boring, cliched outlook certainly has served the Celtics well. The Celtics were dominated by Chicago in transition and on the boards during their two losses at the Garden in games, it should be noted, when the Bulls were able to take the crowd out of the equation by the end of the third quarter.

In the regular season, the Celtics tied with the Toronto Raptors for the best road record in the East, and in this series built on that reputation with a pair of wins in Chicago, capped Sunday when Isaiah Thomas had his best playoff performance to date with 33 points, seven assists and a team-high plus-17.

“Definitely gives us a confidence boost. Evens series out, and go back to square one,” Crowder said of the team’s ability to win back home-court advantage in the process.

Said Horford: “I wouldn’t say that we’re relieved. This is what we set out to do . . . down 0-2. The Bulls took two from us at home and we needed to get them back. We have more work to do.”

Part of the job may involve thinking back to what it felt like for this team when it was still in that 0-2 hole and reports of their demise were being filed across the league, especially by the majority of media types who labeled the Celtics one of the worst No. 1 playoff seeds in the history of the NBA.

Now, the Celtics have a second crack at making home-court advantage work.

“I think it’s simple,” Avery Bradley said. “Our mentality has to be to take care of home and continue to play the right way. We should take pride playing in front of our fans and playing the right way.”

This time around, at home, the Celtics can’t lose composure – a condition exploited by Rajon Rondo in the first two games.

With Rondo out indefinitely with a broken right thumb, the Celtics can once again dictate pace. The Bulls may have found a suitable Rondo replacement in Isaiah Canaan, who turned in some solid minutes in Game 4, but they haven’t kicked out in transition since losing the former Celtic.

And though Thomas continues to grieve over the death of his sister, Chyna, he is once again driving his team.

“I know it helps us feel a lot more confident when he’s on the floor,” Bradley said of Thomas. “Because he’s able to make plays not only for himself but for other guys on our team. And sometimes we need that. Sometimes we need him getting in the paint, kicking it out and hitting the next guy. Because it’s contagious. It helps our ball movement, and when the next unit comes in it’s the same thing. So it’s good for our team.”

The result, according to Bradley, is a team with its old clarity.

“It says a lot about us. It reminds me of what kind of team we were at the beginning of the year, and that’s a team that continues to fight, always playing with a mentality like we’re the underdogs,” he said. “And playing that way, it’s almost like we’re playing with our backs against the wall. And we have to continue to play that way. I know Brad’s going to continue to remind us that. You can’t get comfortable and think this series is over because they’re going to bring it next game.

“We needed to calm down. They were trying to speed us up and force us into some turnovers and force some tough shots. Once we were able to slow down, we were able to get stops on the other end and gain the lead. Isaiah came in the game and made some big plays for our team and got easy shots for other guys. As long as we play that way, it’s tough to guard us.”