Don't call it a comeback.

''Comeback'' doesn't even come close to describing what Britney Spears has done on her new album, arriving in stores Tuesday, her 27th birthday. Not only is Circus the best album of her career, but it also could return her to the kind of massive, multiplatinum sales that she hasn't seen since the turn of the century.

Circus is overflowing with smart, savvy dance pop the kind of hits that she has hinted at in the past with Toxic and I'm a Slave 4 U that creates the perfect escapist soundtrack for these downsized times. All these accomplishments are made that much more incredible considering how low she had sunk personally and professionally last year with her string of tabloid-worthy exploits and the embarra**ment of her clumsy train wreck of a performance on MTV's Video Music Awards.

Her last album, Blackout, sounded so cobbled together and half-done that many wondered if Spears would even be able to record another album, much less one as impressive as Circus. Though she worked with essentially the same producers and songwriters, the results are so different that Spears herself generally known for her choreography and packaging rather than her studio chops must be responsible for the change.

The monster hit Womanizer is only the beginning of the radio-ready barrage Spears is about to throw down. The title track, written by Dr. Luke, is another sleek dance-floor filler, as is the dizzying Mannequin. And she has the disco-influenced Lace and Leather, with its scratch guitar and Chic-inspired bass line.

Lace and Leather is another example of the biggest surprise of Circus the way that it nods to previous classics before taking the songs in a new, contemporary direction. Spears has never been one to look back in her music, but there's a bit of Tina Turner's River Deep, Mountain High on Mmm Papi, a bouncy little multi-culti trifle that telescopes several decades of sugary pop into a little more than three minutes.

Kill the Lights which chronicles her dealings with paparazzi, or maybe one particular paparazzo ex-boyfriend, as she says, ''Is that money in your pocket or are you happy to see me?'' opens with a call-and-response similar to Aretha Franklin's Respect. And on the bonus track Amnesia, one of the album's catchiest tracks, Spears comes over like a cross between the Ronettes and Fergie as she coos, ''I get amnesia when I'm sitting next to you-ooh-ooh.''

It's all part of what seems like a new direction for Spears, one that values musicianship. She tackles some ballads, both from Robbie Williams collaborator Guy Sigsworth, that accomplish their missions. Out From Under shows how her voice, which is still a bit limited, can carry a song, even with spare accompaniment, while My Baby is a bit schmaltzy, as it conveys how Spears is a loving mother now the cornerstone of her image rehab.

In many ways, Circus has Spears switching musical role models. She seems determined to be more like Gwen Stefani, a hitmaker who knows how to use her strengths and weaknesses, and less like Madonna, who thrives on shock value and controversy. Yet somehow, Spears has delivered the biggest shock of all compelling reasons to reverse her cartoonlike tabloid image and take her seriously as a singer.

Forget the release of Chinese Democracy as the year's biggest surprise. Circus is a Britney Spears album I never expected to hear one that matters.