LAS VEGAS — For the last six years, the state of Britney Spears has been summed up best — and most frequently — by the simple phrase she intoned at the top of "Gimme More," from 2007, that has since become a catchphrase: "It's Britney, bitch."

Not a tease, nor a boast, nor a taunt, it's almost apologetic, an apt tagline for a star whose power is self-evident, but also blank and tautological. It's what might appear on the business card of a performer with nothing left to add.

No surprise then that at 32, with more than two decades of performing under her belt, Ms. Spears has already arrived at the laurel-resting portion of her career, landing in a greatest-hits production so winning that it barely needs her at all.

Mostly, she's a pinball during the 90-minute extravaganza "Britney: Piece of Me," her new residency at the Axis Theater at Planet Hollywood Resort & Casino here that had its debut on Friday. Magical things are happening all around her — ornate sets, clever video displays, fiery dancing — but Ms. Spears is there mostly to activate memories, to be a souvenir for the eyes. Rarely did the voice booming out over the speakers appear to be coming directly from Ms. Spears's mouth. Always a notch or three less committed than her backup dancers, she was at times downright listless.

But that's not new: Ms. Spears has long been the pop star most obscured by her own songs. Especially in the second half of her career, since the mid-2000s, the period that followed her tabloid-documented meltdowns, she's been putty for producers, who give her muscular tracks that ask little of her vocally — and even less emotionally — but leave her with an air of power and control.

Judging by the show's narrative, she was invested with that power by dark forces. Early in the night, during the melancholic "Everytime," she was a winged angel falling to earth, swarmed by vampiric dancers in all black when she landed. When they fled, Ms. Spears was remade. Now in a goth dominatrix outfit, she shifted gears to a medley of

"...Baby One More Time" and "Oops!...I Did It Again," the two early hits that cemented Ms. Spears's image as knowing naïf.

This was Ms. Spears at her toughest during this show, which covers about two dozen songs from the whole span of her career, and which she'll repeat some 100 times over the next two years. In general, the set design was more imposing than she was. At points she performed from inside a ring of fire, on top of a rolling pyramid, jumping off a huge prop tree and under a sheet of water falling from the ceiling. Even during "Freakshow," when she had an audience member — in this case, the "Extra" host Mario Lopez — bound in a harness so she could walk him like a dog, she came off playful, not salacious. (Ms. Spears then signed a T-shirt for him.)

"Britney: Piece of Me" comes on the heels of Ms. Spears's energetic but rarely inspiring eighth album "Britney Jean" (RCA), which sold only about 107,000 copies in its first week, the lowest opening of her career. She may have the name recognition of a global superstar, but not the drawing power she had even a few years ago. Performing here, in this 4,600-seat hall, is a relatively low-risk proposition, and doesn't demand that she extend her relevance. (The first single from "Britney Jean" was "Work Bitch," a nod to "It's Britney, bitch.")

This is also a transitional moment for Las Vegas, a town becoming less reliant on older-audience-skewing musical revues and leaning more heavily on nightclubs. Ms. Spears's show is a midpoint between the then and now, a legacy act with cross-generational appeal offering a show that might as well have been run by a D.J. (This newly renovated theater, too, is a hybrid, with two standing-room pits and a row of V.I.P. bottle-service tables abutting the stage's lip.)

"Piece of Me" is probably the least staid of the single-artist Vegas residencies; everything about it, save Ms. Spears, is splashy and top volume. The costumes, by Marco Marco, were vibrant, and the choreography, by the Squared Division, was powerful, particularly during "Scream & Shout," when dancers maneuvered a pair of circular hamster-wheel-like structures. As for Ms. Spears, the version of her displayed on screen — from old videos and the like — almost always looked more confident and more comfortable than the Ms. Spears who was onstage.

That contrast was only heightened by the fact that throughout the night, one of Ms. Spears's disciples, Miley Cyrus, was at a front row V.I.P. table, dancing enthusiastically and singing along. If Ms. Spears is one of the last Stepford pop stars, Ms. Cyrus is a new model — unpredictable, self-determining, actually fun.

Ms. Cyrus has long pledged loyalty to Ms. Spears as her childhood idol, and even collaborated with her on a song from her Ms. Cyrus's album, "Bangerz" (RCA), though Ms. Spears sounds robotic, especially up against Ms. Cyrus's natural effervescence. But Ms. Cyrus's presence in Las Vegas — she was one of a handful of celebrities at this show, along with Katy Perry and Selena Gomez — wasn't wholly to display her devotion to Ms. Spears.

Later that night, she hosted the opening of Beacher's Madhouse at the MGM Grand, an extension of the rowdy Los Angeles post-vaudeville nightclub of the same name. What had seemed, up close, like a night for Ms. Spears's coronation as the latest of this city's marquee names was in fact just a prelude. Ms. Spears may have hosted a cool party, but the night went on without her.