By Matthew Geagan, CBS Boston

BOSTON (CBS) — The Boston Celtics are the top seed in the Eastern Conference heading into the playoffs. While the team has showed improvement in each season under head coach Brad Stevens, 53 wins and the top seed wasn’t what most were predicting when the season began.

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Even the players figured the Cleveland Cavaliers, the defending NBA Champions, would be on top of everyone else in the conference when the regular season came to a close. Minus Isaiah Thomas, who despite his 5-foot-9 frame, exudes more confidence than many of the giants in the NBA.

“I did [expect this] but I’m different than others,” Thomas said with a smile after Boston wrapped up the regular season with a win over the Milwaukee Bucks on Wednesday night.

The Celtics have improved every regular season under Stevens. After winning just 25 games in his first year out of the college ranks, they tallied 40 wins in his second season and made the playoffs as the 8-seed, mostly because of Danny Ainge’s deadline day deal for Thomas. They won 48 games to finish the year 14 games above .500 last season, and now they’re a 53-win team and the top dogs in their conference.

What was supposed to be a lengthy rebuild focused around a collection of high draft picks stolen from the Brooklyn Nets has been put into hyper drive. But there’s one thing that has alluded the team their last two campaigns, and it’s really what matters most: Postseason success.

Boston was swept out of the playoffs by the Cavs two seasons ago, and endured a disappointing six-game loss to the Hawks in the first round last season. Last year’s postseason defeat mostly accepted among the Celtics faithful. Despite those 48 wins in the regular season, the Celtics clearly weren’t ready for the NBA’s big stage.

This year is different. This year, the expectations are real, and the Celtics are prepared to live up to them.

“We want to win. We’re a totally different team than we were last year and the year before,” said Thomas. “We have some news pieces and are playing at a higher level. We want more. Previous years we wanted to make the playoffs and see what happened. This year we want to advance as far as possible.”

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“I think a lot of teams in the past were happy to be in the playoffs. We’ve got a much different feeling in the locker room,” said forward Jae Crowder. “Playing on those teams, it’s just a much different mentality. Each and everybody wants to actually win.

The 1-seed in the conference is special, but the Celtics aren’t celebrating. They know the real season begins on Sunday night, and their focus is now on the Chicago Bulls. Boston split their season series with Chicago 2-2, and they know from past experience that nothing can be taken for granted in the playoffs.

“They’re a good team with good players and they have a great bench. They’ve had their ups and downs, but every team has their ups and downs,” guard Avery Bradley said of Chicago. “They are playing great basketball and we’re going to play a good team.”

“It’ll be a good matchup for us. They’re the 8-seed, but they’re better than that,” added Thomas. “They’re a really good team, especially with Dwyane Wade back.”

The Celtics are a team built on confidence, and even with their finish atop the conference, they know they still have a lot to prove. They aren’t looking passed the first round, but it’s clear the world expects the Cavaliers to get the best of them if the two teams are fortunate to meet in the Eastern Conference Finals. It’s that underdog mentality that has fueled the team throughout the season, and they’re ready to prove they can win when it matters the most in the NBA.

“We know the bigger picture. We have a lot of upside and we feel we haven’t hit our stride yet,” said Crowder. “Hopefully we hit it during the playoffs at the right time.”

“Once you get into the playoffs, the seedings kind of go out the window. You have to take care of business and be locked in from the jump,” said Thomas. “We have to win, by any means. No matter how I play, I have to figure out a way to lead this team and win playoff games. That’s my only focus and my only goal.”

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Boston’s remarkable regular season is a testament to the hard work the players and coaches put in every day, but it won’t mean much if the Celtics can’t win in the playoffs. They know this and they accept it. Now it’s time to take things to that next level.