Britney Spears dropped in on the Backstreet Boys before the band's show at the Pechanga casino in California on Friday night (25Jun10).
Spears, a longtime fan of the Everybody (Backstreet's Back) hitmakers, made an unannounced appearance backstage before the concert to greet the group.
She then joined the sold out crowd to cheer the pop stars on.
It has been quite a week for the boy band - they were joined onstage at New York City's Radio City Music Hall last weekend (19Jun10) by fellow ageing pop hunks New Kids on the Block.