Christina Aguilera Credit: Mark Seliger/NBC

Christina Aguilera may be sitting on the sidelines during the fourth season of The Voice, but the outspoken pop star is hardly keeping quiet. Ahead of the show's March 25 season premiere, the "Just a Fool" singer pleaded with returning mentors Blake Shelton and Adam Levine to be on their best behavior.

"[I] know how you boys can get," Aguilera tweeted March 18, "so watch [the] locker room humor and treat Shakira like lady she is on [the] new The Voice." The five-time Grammy winner, 32, also teased birthday boy Levine, celebrating his 34th birthday that evening. "I know you'll be whipping out tequila shots tonight," she wrote. "You turn 40, right? Hehe. You know I'll always bust your balls."

Shakira, who welcomed son Milan with boyfriend Gerard Pique in January, isn't the only new face on The Voice this season. Usher, 34, now sits in the swiveling chair previously filled by Cee Lo Green, 38.

As in previous seasons, The Voice is broken into four stages: blind auditions, battle rounds, knockouts and the live performances. The blind auditions were taped in the fall of 2012, while Shakira was still pregnant.

During a recent interview with Entertainment Tonight, Shakira, 36, explained how she's able to relate to the show's aspiring singers. "I started out really, really young," the Colombian musician said. "I've been doing this for 25 years, and it hasn't been a path of roses. . . I know what it's like to feel that fear and the need of affirmation and appreciation. To build confidence in yourself is the toughest thing."

The Voice, hosted by Carson Daly, returns Monday, March 25, at 8 p.m EST on NBC. The winner will be revealed live on June 18.