With uncertainty over which celebrity judges will return for Season 4 of The Voice in Spring 2013, NBC has reportedly been negotiating with R&B singer Usher and Latin crooner Shakira to replace Christina Aguilera and Cee-Lo Greene.

The singers are very familiar with NBC's reality show format; Usher and Shakira have both appreared on "The Voice" previously as guest coaches for Adam Levine and Blake Shelton.

Aguilera will release her new albm, "Lotus" in November and reportedly has said she is not sure if she can committ to another season.

Aguilera told Billboard magazine that "The Voice" has "kept me away from my heart, which is creating my own music."

"It's been a joy to be a part of other people's journey, to be able to inspire and be a part of new singers coming up in this business," she added, according to Us Weekly. "But I was starting to get really worried and concerned that I wouldn't have the time to go and be an artist again."

Aguilera added that her interest in leaving the show would be temporary and that if she left she would "probably be back. I just need to do my thing for a minute then I can come back and be that much better of a coach. I just need a second to get back to me."

"[I thought], 'It'll keep me close to my son [Max, 4] in L.A. I don't have to travel," the five-time Grammy Award winner said. "Then it kind of took off, and there goes my free time!"

NBC declined to comment on the reports but executive producer Mark Burnett reportedly "made it very clear that these chairs are always our chairs."

Paul Telegdy, the network's president of alternative and late night programming told The Hollywood Reporter that "a chair on the panel is not a job for life, nor should it be."

"The Voice" is in talks with the award-winning singers Usher and Shakira and will be looking to scoop popular names such as theirs to combat intensive competition from other reality TV singing shows such as American Idol and The X Factor.

On Sunday, Sept. 16 Fox announced it completed its lineup for the judges' table for "American Idol" Season 12. The show added Nicki Minaj and Keith Urban to Mariah Carey, who was confirmed in July and Randy Jackson - one of the show's original judges.

Last week's premiere episode of Season 2 of "X Factor USA" featured two new celebrity judges - pop singer Britney Spears and Demi Lovato, who both have received widespread praise for their showings so far.

"The Voice" Season 3 is currently airing Mondays and Tuesdays at 8/7C on NBC.