Abdul made her first appearance on the program since departing in 2009, looking sexy in a red dress and complimenting finalist Candice Glover on her unique rendition of her 1989 hit Straight Up a night earlier.
Paula, commenting on the show's first all-female set of finalists, said, "Congratulations to America for getting it right!"
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Aiken, who was runner up to Ruben Studdard 10 years ago, took to the stage to sing a heartfelt version of Simon & Garfunkel's Bridge Over Troubled Water.
"I never thought I'd do what I'm doing now and this show is the reason I'm able to do that," Aiken said.
Season three winner Fantasia delivered an emotional performance of her latest release, Lose to Win, that had Nicki Minaj welling up. The judges gave her a standing ovation for her efforts.
Also spotted at the taping was rock group Fall Out Boy, with lead singer Pete Wentz saying, "We're super excited to be here."
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American Idol continues next Wednesday on Fox at 8/7c.