Lopez and Lambert will be among more than a dozen performers at Sunday's ceremony, and they took time out Thursday to perfect their performances.
Lopez is in top form, but she's still a little nervous. She'll be singing a song fans have never heard before and performing live on TV for the first time in years.
"You know what you're doing and you feel very good about it, but at the same time, you wouldn't be human if you're not afraid," she said after her rehearsal.
She's set to perform "Louboutins," the first single from her new album, "Love?" - due in stores next year. Flanked by backup dancers, the 40-year-old entertainer busts out her Fly Girl moves in a number that begins in a boxing ring and ends in the audience. Famed sports announcer Michael Buffer is in the act, too.
"I think it's going to be good," she said. "Dancing again, singing, the lights and the costumes - it all feels like second nature."
Veteran choreographer Kenny Ortega, director of "Michael Jackson's This Is It," gave Lopez's performance a positive review.
"Awesome," he said after dropping by the Nokia Theatre to watch her rehearse.
Lambert, 27, said he's eager to see how the star-studded crowd responds to his provocative performance, a sexed-up rendition of "For Your Entertainment," the first single off his upcoming album of the same name.
"I can't wait to see people I look up to as artists in the audience watching. I think that will be a trip, and exciting and motivating all at the same time," he said after his rehearsal. "It will be really interesting to see what kind of reaction the number gets. It's different. It really does have an edge to it."
The performance is heavy on leather and chains and includes Lambert dragging a shackled woman across the stage.
Other artists set to perform at the 37th annual American Music Awards include Rihanna, Whitney Houston, Green Day, Lil' Wayne, Lady Gaga, Eminem, Keith Urban, Jay-Z and Carrie Underwood.
Janet Jackson will open the show with a medley of songs, but producer Larry Klein wouldn't say whether her performance would be a tribute to her late brother.
"It's a surprise," he said.
Fans voted online to choose the winners of the American Music Awards, which honor the year's top-selling artists in eight popular genres. The ceremony will be broadcast live on ABC from the Nokia Theatre.