Whoever thought Britney Spears had lost it ... boy, were you ever wrong.

The 29-year-old Mississippi-born beauty, a mother-of-two, showed off her incredible figure -- particularly her derriere -- in front of 5,000 fans at San Francisco's Bill Graham Civic Auditorium on Sunday. Part of the event will be broadcast on Good Morning America Tuesday, as host Robin Roberts, fresh off her testy chat with Chris Brown, joined Spears onstage at one point.

The resurgent chart-topper changed costumes several times during the event, wearing luscious get-ups like a a silver sequined body-suit