"I've got this chronic back problem," he said during Monday's episode. "I've had a wonderful, brilliant, fantastic blast of a time ... Pure joy. And I want to thank every one of you folks for your support."
The 77-year-old actor was the oldest contestant during the 18th season of DWTS, and while he made it through the first round of eliminations on March 24, his score began to plummet after the second round.
"I look like an old man trying to be a young person," he said of the Star Wars-themed cha cha cha he performed during week two.
In spite of getting the lower score for two weeks in a row, the judges encouraged Williams to step it up if he wanted to stay in the competition. However, his injury kept him from even trying.
"My time on Dancing With the Stars is an epic moment," he said Monday. "This is something I'm going to carry with me."