Oakland, Calif. ? The Detroit Tigers and Oakland Athletics advanced to the AL championship series and celebrated as if they’d won a World Series.

With good reason.

The A’s finally ended what had begun to feel like a hex of first-round failures by sweeping the Minnesota Twins. The Tigers, still hearing all about the club’s 119-loss showing from three years ago, overcame a late-season collapse that cost them the AL Central, then defeated the not-so-mighty-after-all New York Yankees.

Now, these teams must put the thrill of just getting past the first round behind them to focus on a best-of-seven series that both sides expect to be hard fought and without a clear-cut favorite. Game 1 is tonight, with Barry Zito pitching for the A’s against fellow left-hander Nate Robertson.

“For us, it’s just about continuing on that high,” A’s first baseman Nick Swisher said Monday. “So they had a little down time at the end of the year – so what.”

Both teams held wild clubhouse parties in their home ballparks in a two-day span last week – first Oakland on Friday, then Detroit a day later.

“This is a loose ballclub. We’re happy to be here, but we’re here to win,” Oakland slugger Frank Thomas said. “We’ve stressed that throughout the last of the season and the playoffs so far. … They were the biggest surprise in baseball all year long. We know what to expect. They’ve earned it.”

The Tigers went all out in the season’s final game to try to win the division to avoid opening the playoffs against the Yankees, using their top relievers and even bringing in starter Kenny Rogers out of the bullpen. They still ended up being swept by the Kansas City Royals and settling for the wild card.

Yet it worked out just fine for first-year manager Jim Leyland’s team, which dropped Game 1 to New York before winning three straight.

“The sweep for the A’s against the Twins is history, and the great series win for us against the Yankees is history,” Leyland said. “Now this is a new chapter, which is what the playoffs are all about. … Both teams are going to come out with confidence. Both teams have reason to be confident. Both teams had very good regular seasons. Both teams obviously had a very good playoffs so far.”

The free-swinging Tigers went 5-4 against the A’s this year with a 52-39 advantage in runs scored, marking the first time since 2000 Oakland lost the season series. Detroit was one of four AL clubs the A’s had losing records against.

“Them being here is not an accident,” A’s manager Ken Macha said. “They’re pretty darn good. They deserve to be here. Our guys feel good about themselves, and they should. We won the division and played three great games against Minnesota.”

This Detroit team, with eight active holdovers from the awful ’03 squad, has shed the loser label that followed the club everywhere in the Motor City and elsewhere.

Even walks down the street would bring insults, third baseman Brandon Inge recalled.

“Once you have a season like that, there’s really only one place to go. You can’t get much worse than that,” Inge said. “That season to a lot of us doesn’t even exist. I feel like we’ve come back and accomplished much more than what that season kind of gave us a reputation for.”