"We decided - recently - we're going to make this a top 13," Cowell revealed.
The three other wild-card finalists were Jasmine Murray, the big-voiced 16-year-old high school student from Starkville, Miss.; Megan Corkrey, the quirky 22-year-old single mother from Sandy, Utah; and Matt Giraud, the soulful 23-year-old dueling piano player from Kalamazoo, Mich.
The previously dismissed foursome were selected by the show's judges.
Desai's last-minute selection wasn't the night's only seemingly unscripted moment. After oh-so-emotional semifinalist Tatiana Del Toro performed Whitney Houston's "Saving All My Love For You," she kneeled and pleaded for a spot from the judges. Then, host Ryan Seacrest came on stage and told her she didn't have to get up off her knees.
"Family show, family show," judge Kara DioGuardi repeatedly proclaimed throughout the awkward moment.
Desai, Murray, Corkrey and Giraud join previously picked finalists Kris Allen, Danny Gokey, Alexis Grace, Allison Iraheta, Adam Lambert, Scott MacIntyre, Jorge Nunez, Lil Rounds and Michael Sarver. The 13 finalists will start competing next Tuesday - with one singer sent packing each Wednesday.