Britney's musical chairs: Rated M for Mature!

Britney Spears blazed back onto the stage on Tuesday's Good Morning America during a pre-taped show in San Francisco -- her first performance before a live audience in two years (since her massive Circus world tour).

For her much-anticipated comeback, Spears, 29, slipped on a skintight, sequined bodysuit as she shook her famous body to "Hold It Against Me," the smash #1 from her new album Femme Fatale, out Tuesday.

Watch Spears bust a move -- and get seriously sexy in a chair as backup dancers help her out -- in this clip.