The last time Marvin Williams experienced postseason basketball he was 18 years old, wearing Carolina blue, and one of the most dominant players on the court. With the NBA in fingertip’s reach but not yet a reality, he became a champion, joining the likes of Michael Jordan, Dean Smith and James Worthy in a family of NCAA title teams at the University of North Carolina.

Williams’ Tar Heels defeated Illinois, 75-70, in the 2005 NCAA title game.

Three years later, with the ‘05 championship in his rearview mirror, Williams is in the postseason again, this time as a budding NBA star competing against the league’s elite — trio Kevin Garnett, Paul Pierce and Ray Allen — and the No. 1-seed Boston Celtics in a first-round playoff series.

“It was huge,” Williams said Tuesday during a phone interview, describing his emotions during Game 1 of the No. 8 Hawks’ seven-game series against Boston. “It reminded me of college. The intensity level was so high.”

Boston took the game, winning 104-81 for a 1-0 series lead.

“It’s a tough match-up for us, no question,” Williams said, managing only nine points on 2-of-7 shooting in Game 1.

Ending the season at 66-16, a 42-game turnaround from last year’s 24-58 team, the Celtics won 29 more regular season games than Atlanta (37-45), a startling stat.

“They’ve got a lot of veterans, guys with (playoff) experience,” Williams said, alluding to Boston’s Garnett-Pierce-Allen trio, a three-headed monster of all-star talent who helped the Celtics to the biggest win-loss turnaround in NBA history.

Williams scored 13 points and pulled down four rebounds two nights later in Game 2, but Atlanta lost again, this time 96-77. Unable to knock down shots, the Hawks failed to make a three-pointer and ended 23-of-60 from the field.

But even with Atlanta trailing the series, down 2-0, Williams has reason to remain optimistic as the Hawks still have a chance to climb back into the series.

“In college it’s single elimination,” he said of the playoff format. “I’m happy we at least get four games and, hopefully, seven.”

The series now shifts to Atlanta — Game 3 tips off at 5 p.m. today — and Williams and Co. will have the fans’ support.

“They’ve been great to us all year, everybody’s excited,” Williams said of the Atlanta fans, who haven’t enjoyed playoff basketball since the 1998-99 team reached the postseason, losing in the second round to the New York Knicks.

The nine-year Atlanta playoff layoff means Williams isn’t the only Hawk arriving to the postseason scene.

“The previous two years we didn’t make (the playoffs),” he said, referring to the team’s youth.

Rookies Al Horford (Florida), Acie Law (Texas A&M) and Mario West (Georgia Tech) join Williams, Josh Childress, Solomon Jones, Jeremy Richardson and Salim Stoudamire as playoff newcomers.

Regardless of what happens in the Boston series, Williams will walk away from his third season in the league a stronger player.

“The best way to learn is to get out there and play,” Williams said, his points, rebounds and free-throw percentage all up from last year. “I just got a little older.”

Working to complete his undergrad degree from UNC, Williams said he will spend the summer traveling between North Carolina, Atlanta and Bremerton, balancing summer courses and training while finding time to spend with his family.

“I’m not sure if I’ll make it back to Bremerton before summer classes,” he explained. “I’ll be in and out (of Bremerton), visiting friends and family.”

Citing life away from basketball, Williams said he’s looking forward to coming home to Bremerton to see family and friends.

“Of course they’re watching (the playoffs), but with (family), it’s stuff outside basketball,” he said. “That’s what I love about family… my little brother, my mom, my grandma, they all mess with me.”

In his most recent visit to the Pacific Northwest, Williams poured in a career-high 33 points in the Hawks’ 99-90 victory against the Sonics at Key Arena, possibly the final time he’d face his boyhood team in Seattle.

“It’s sad to see them go. It’s gonna be a major blow,” he said of the Sonics’ now likely departure. “Growing up with a team and seeing them go, it’s tough.”

Along with the stellar on-court performance against Seattle, witnessed by more than 45 friends and family, Williams also had a highlight year away from the hardwood, earning the Jason Collier Community Service Award, an honor handed to the Hawk player who shows the most involvement to the Atlanta community.

Williams works with the Atlanta chapter of the Starbright Children’s Foundation, an organization that provides entertainment and education to seriously ill children whose lives are in danger. He also hosts special “meet-and-greet” sessions on game-days to sign autographs and snap photos with kids who sit in his “MVP” section.

“That was gratifying,” he said, “to be recognized for your hard work and time.”

The teams battle again Monday at 5 p.m. in Game 4 while the Game 5, if necessary, is scheduled for Wednesday. Visit www.nba.com for up-to-date playoff information.