Rademacher and partner Kym Johnson were eliminated in the semifinals. On Monday, the duo received a 24 out of 30 for their Samba and 27 of 30 for their Charleston.
The actor said he knew it was his time to go.
"My knees are shot from all the Jiving and all the Charlestoning [sic]," he said. "I don't think I could have done another week."
Johnson said she was proud of her partner and that they had a lot of fun along the way.
"He did an amazing job," she said.
Disney star Zendaya Coleman and her partner, Val Chmerkovskiy, survived elimination.
The show hosted a party to mark the episode milestone, inviting contestants from its past and present.
"It blesses so many people. It's one of those experiences that you will never forget," said singer Brandy, who appeared in Season 11.
The remaining four couples will face off in the final Monday.