EXCLUSIVE: Katie Holmes' SYTYCD Dance: All the Details

For her routine, the actress, 30, sings Garland's "Get Happy," from her 1950 musical, Summer Stock, a witness at the Monday rehearsal at FOX Studios tells Usmagazine.com.

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Holmes also dons an all-black ensemble similar to Garland's: black velvet fedora, patent leather heels, nylon stockings, body-hugging skirt and micro-mini skirt.

Part of the set appears to be made from scaffolding, creating the illusion that she's dancing on a rooftop with a sky backdrop. She shares the stage with eight male dancers, who wear matching suits and ties.

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During the number, Holmes -- who practiced by lip-syncing to a backing track -- twirls her hair and "seductively shakes her hips," the witness tells Us.

For her finish, Holmes tosses her fedora in the air, and one of the guys catches it. She's also lifted over three of the dancers' heads at one point.

Holmes is being directed by none other than Nigel Lythgoe, the show's producer and judge. Pal Tyce Diorio choreographed the routine.

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On Monday, Holmes and the dancers practiced for hours, taking few breaks. They called it a night at 6 p.m.

The actress -- who also danced during a guest appearance on Eli Stone -- "is killing it," a source told Us. "She looks incredible. She is just doing an amazing job. Everyone is absolutely floored by how talented she is."

Her performance "might broadcast this week as news has started to leak out -- but they're trying to save it for July 8th, as that is the 100th episode," the source continued.