Spears Kin Pump-Up Brit for 'Circus'

Britney Spears is anxious about taking the stage tonight in New Orleans for her first concert since 2004's "Onyx Hotel," but according to the Spears family, the pop princess has nothing to worry about!

The "Womanizer" singer gave a private preview performance of "The Circus Starring Britney Spears" for 30 family and friends -- and her nerves reportedly melted when they loved the show!

"Britney was really tense up until the show," a Spears insider dished to the NY Daily News, "but she got such positive feedback that she's a lot more confident about her performance."

The whole Spears family -- including the divorced Jamie and Lynne, sister Jamie-Lynn and niece Maddie, and Britney's babies Sean Preston and Jayden James. "Jamie and Lynne don't get along at all, but they behaved," the source revealed of Britney's parents. "They're really rooting for Britney and wanted to make the evening all about her."

Ex Kevin Federline also flew to Louisiana to be with Brit and his sons -- and while K-Fed didn't partake in the group activities, he will reportedly be on hand to watch his tots as Spears takes the stage.

Ms. Spears joined pal Madonna onstage during her "Sticky and Sweet" tour -- and a source revealed to the paper that Britney is rumored to be returning the favor! "The rumor going around is that Madonna is going to join Brit onstage for at least two tour dates -- and that would be fierce."

Britney's "Circus" kicks off tonight at the New Orleans Arena.