There have been moments, sometimes lasting years, when Britney Spears has seemed like an accessory to her own career - the public face of a vast army of producers, songwriters, choreographers, stylists, publicity agents, and marketing executives. But holding together that tangle of competing agendas is no easy task. Part of what great pop stars do is provide a sense of continuity, using the force of their personality to bridge the gaps between trends.

It cannot be an easy job, and six weeks into her Femme Fatale tour, even Britney Spears looked as though she was tired of being Britney Spears.

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Her 85-minute show at the Wells Fargo Center on Saturday night, already brief by arena standards, paused several times for costume changes and dance interludes.

And when Spears was on stage, she appeared to be lip-synching much of the time.

Spears' fans, who, judging from the crowd, are still numerous and highly engaged, contend there is no way for her to sing and perform the elaborate dance routines that are part of her act.

But even stipulating that singing and dancing are mutually exclusive pursuits, which has not historically been the case, Spears' moves lacked precision and snap. Minor glitches, such as wobbly set pieces or a sequined jacket that snagged when it should have pulled off smoothly, marred the illusion of a well-tuned spectacle. And even though Spears pulled an audience member on stage for a lap dance during "Lace and Leather" and invited a few dozen for a dance-off on "Womanizer," she never engaged the crowd in more than pro forma banter.

When she grabbed a seat and turned on her microphone for the ballad "Don't Let Me Be the Last to Know," she proved she is still eminently capable of putting a song across. And new songs such as the dance-floor anthem "Till the World Ends" and the double-Dutch ditty "How I Roll" are as catchy and insinuating as anything she has recorded.

But she seems as though she is playing catch-up with more vital artists, such as opener Nicki Minaj, whose sci-fi-themed set drew deafening applause.

Minaj, who earned a one-song encore of her own, reappeared at the end of Spears' set for a guest rap, but only as a virtual presence on the video screens. Perhaps Minaj was just putting her high heels up after a high-energy performance, but it might just have been Spears' way of putting the competition in her place.