Britney Spears Gets 'NSYNC Birthday Serenade

If you want to be serenaded by the members of 'NSYNC on your birthday, it helps to be connected — like, really connected.

Britney Spears celebrated her 19th birthday Saturday night by joining the group onstage at the San Diego Sports Arena. The teen queen first hit the stage incognito, wearing a wig and glasses, during the boy band's performance of Johnny Kemp's '80s dance hit "Just Got Paid," according to a source close to the production.

'NSYNC then asked the crowd to join them in singing "Happy Birthday" to the young pop star, who also attended the group's Friday night show in San Diego but did not take the stage.

The San Diego shows afforded the pop outfit even more reason to celebrate as they marked the end of 'NSYNC's fall U.S. tour, which had been rolling along since mid-October. A spokesperson for the venue said the group and its tour crew held a private end-of-tour party after Friday night's show, where the group also serenaded Spears with a rendition of "Happy Birthday."

The shows also marked the latest high-profile joint appearances by Spears and 'NSYNC, and specifically pal Justin Timberlake. Spears joined Timberlake at last week's My VH1 Music Awards, and both Spears and 'NSYNC are slated to appear at Tuesday's Billboard Music Awards.