Citing a chronic back problem and medical advice, actor Billy Dee Williams withdrew from season 18 of ABC's "Dancing With the Stars" tonight.

The "Star Wars" actor thanked his pro partner, Emma Slater, and said he'd always remember his time on the show.

"And I want to thank every one of you folks who've been supporting us," he said, gesturing to the viewing audience, which gave him a standing ovation.

The 77-year-old - who has had hip surgery - has been at the bottom of the leaderboard. Judges acknowledged his struggles on the dance floor even as they praised his spirit.

In a departure from tradition, no one was voted off the dance floor tonight.

Tonight's show featured "Good Morning America" co-host Robin Roberts as a guest judge, alongside head judge Len Goodman and judges Carrie Ann Inaba and Bruno Tonioli.

Roberts said she was a longtime fan of the show and thrilled to be on the panel.

"To experience this electric atmosphere first-hand is remarkable … thank you," she said.

The routines in tonight's show were dedicated to the most memorable year of each stars' life.

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Tonight's Routines

James Maslow and Peta Murgatroyd: The singer/actor's jive was dedicated to the year that he overcame bullying in high school. Judges loved it. "What's going on? Was that an earthquake, an aftershock, or did you just rock this place?" said head judge Len Goodman. Carrie Anne Inaba called the routine "vibrant" and "kinetic," and Bruno Tonioli said the performance had "all the explosive energy of a lightning strike," adding, "I cannot believe what you've achieved in three weeks." Roberts said she appreciated the way Maslow attacked every dance, and said his example would touch a lot of young people who were bullied. Maslow earned four 9s from judges for a total of 36 out of a possible 40 points.

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NeNe Leakes and Tony Dovolani: The reality TV star's tender rumba with her shirtless partner evoked 2013, the year she remarried her husband after previously divorcing him. Leakes earned a standing ovation from the audience - which included her cheering husband. Roberts told Leakes that she had an enormous personality but had allowed people to see a different side of her tonight. "It was so sensual and beautiful," she said. Tonioli said Leakes maintained the flow of movement well "for a non-dancer," but said she needed to improve her basic rumba step. Inaba said Leakes did a "beautiful job." Goodman said he enjoyed watching it. "The minute I saw the bed, Tony half naked, NeNe with no shoes on, I was expecting the worst, but I was pleasantly surprised," he said, speaking of the set design and Leakes' and Dovolani's costumes. Leakes earned three 8s and one 7, for a total of 31 points.

Cody Simpson and Witney Carson: Simpson dedicated his jazz routine - set to his own song, "Surfboard" - to his family, who supported his dream and moved from Australia to Hollywood so he could further pursue his career. He won high praise from the judges. Roberts commended him for being respectful of the dance but making it his own, and Tonioli said the freshness he injected into the routine was typical of the evolving essence of jazz. Goodman said it was "well done," while Inaba said it was "fantastic." She praised his "vocabulary of movement," adding: "I was, like, eating it up." Simpson earned three 9s and one 8 for a total of 35 points.

Danica McKellar and Valentin Chmerkovskiy: The "Wonder Years" actress's poignant contemporary routine recalled the year that her son was born and her grandmother died. "That was just the most beautiful, sincere, delicate performance that I've ever seen," a visibly moved Inaba said. Goodman said McKellar's movement was "so fluid, it was like butter on a hot muffin." Tonioli said: "It was enchanting motion driven by emotion. It was spellbinding," while Roberts told McKellar she was "indeed a wonder and a sheer delight." McKellar earned four 9s for a total of 36 points.

Drew Carey and Cheryl Burke: The TV game show host's waltz recalled 2005, the year his son was born and he was motivated to lose weight and improve his health. Goodman thought the dance was "charming" but became "muddled," and Inaba said she could tell how much the routine meant to Carey but said that his face seemed stressed because of his intensity. Roberts called Carey a "pleasant, marvelous, beautiful surprise this season," and Tonioli said there was plenty of waltz content. Carey earned two 7s and two 8s for a total of 30 points.

Meryl Davis and Maksim Chmerkovskiy: The Olympic gold medalist's ethereal foxtrot drew high praise, not just for Davis but for her pro partner's choreography. Roberts called it "beautiful to watch," and Tonioli said the routine was "irresistibly romantic … the choreography, I have to say, Maks, was extraordinary." Inaba said Davis and Chmerkovskiy had a good partnership, and Goodman finished: "I tell you, Meryl and Maks, M and M, sweet and delicious. Well done!" Davis earned three 10s and a 9, for a total of 39 out of 40 points.

Amy Purdy and Derek Hough: The paralympian snowboarder's touching contemporary dance recalled the first time she danced on prosthetics after losing both her lower legs to bacterial meningitis. The routine was her way to express thanks to her father for donating a kidney to her, and Tonioli called it "heartwrenching" yet "inspirational and life-affirming." Inaba said Purdy reminded people about what it truly means to be human. "You are just so inspiring," she said. Roberts said there weren't enough superlatives to describe Purdy, but said she hoped that Purdy knew how much she was inspiring others. Goodman added: "It touched my brain with the level of difficulty and it touched my heart with the level of artistry. It was truly beautiful. Well done." Purdy earned four 9s for a total of 36 points.

Charlie White and Sharna Burgess: The Olympian's bubbly jive - set to Pharrell's hit, "Happy" - won rave reviews from the judges. "That was fantastic. I love the way you worked the room," Inaba said, although she said faulted him for being ahead of the music twice. Roberts called the routine "a celebration. You make us so happy. You have from day one," she said. Goodman quipped: "You were steaming, I was beaming; it's happy hour … we're all happy." Tonioli said the routine was cheerful, uplifting and delightful. White earned for 9s for a total of 36 points.

Candace Cameron Bure and Mark Ballas: The "Full House" actress's jive recalled the year her hit sitcom ended and she got married. Goodman said he thought the routine was sharp and clean and precise, and Roberts added: "You were snazzy, you were hitting it, it was wonderful to watch." Tonioli said it needed to be a bit cleaner in the footwork but said she "let it loose." Inaba called Bure's performance "a great jive," but advised her to be aware of how she was holding her shoulders. Bure earned four 8s for a total of 32 points.

The Judges' Scores

Meryl Davis and Maksim Chmerkovskiy: 39

Charlie White and Sharna Burgess: 36

Amy Purdy and Derek Hough: 36

James Maslow and Peta Murgatroyd: 36

Danica McKellar and Valentin Chmerkovskiy: 36

Cody Simpson and Witney Carson: 35

NeNe Leakes and Tony Dovolani: 31

Drew Carey and Cheryl Burke: 30

Candace Cameron Bure and Mark Ballas: 32

Billy Dee Williams and Emma Slater: Due to problems with his back, Williams withdrew from the competition.

(Getty Images)