Britney Spears and Christina Aguilera, both former Mickey Mouse Club stars, began their careers together in the early 1990s and later achieved chart-topping success in the late '90s.
During that period, the media often pitted them against each other in a manufactured pop diva rivalry.
Jimmy Kimmel asked Aguilera if she hopes to make a cameo in Spears' upcoming tell-all memoir, "The Woman in Me."
Aguilera responded with uncertainty, stating that she doesn't know and is hoping everything is good with Spears.
When asked if she'd prefer to be in the book or not, Aguilera hesitated and playfully suggested that Kimmel should be in it instead.
The book, set to be released on October 24, covers Spears' 13-year conservatorship, her famous head-shaving incident, and her relationship with her estranged father, Jamie Spears.
Aguilera also discussed her family's elaborate Halloween decorations and her upcoming Las Vegas show at the Voltaire Belle de Nuit at the Venetian Resort Las Vegas, starting on New Year's Eve weekend.