Billy Crystal is ready to go one final round with friend and foil Muhammad Ali.

No comic on stage could float like a butterfly and sound like Ali quite like Crystal.

Often imitated? That was Ali. His boisterous voice roared for decades, he would boast he was The Greatest, before it turned soft, raspy and silent as he battled Parkinson's.

Ali in his heyday was ripe for impression -- and comics and sketch comedy shows all took aim at the champ.

Long before movie stardom, Crystal was still a relative unknown in the early 1970s when he was picked as part of a dais that would help roast Ali at a banquet. Crystal's Ali routine -- he also mimicked longtime Ali foil Howard Cosell -- would become a part of his act for the rest of his career.

Crystal is among the mourners scheduled to speak Friday at Ali's funeral. President Bill Clinton will deliver a eulogy.

It's easy to imagine Crystal will break out bits of his old routine and make an arena full of fans, friends and family feel for a moment that Ali is talking to them one more time. Ali's voice had been diminished for so long, that it was often modern-day artists who would introduce a new generation of fans to the boxer's quotes and quips through comedy.

Crystal remembered Ali last week on Twitter with a video of his 1979 performance called "15 Rounds ," with the caption, "For the greatest man I have ever known."

Crystal's nearly 12-minute performance at Ali's 1979 retirement tribute show at the Great Western Forum captures both at their best: Crystal breaks out his impeccable Ali ("I'm so fast, I can play pingpong by myself"); The Greatest can't stop laughing and later rises for a standing ovation.

Here are some of the memorable impressions of the wannabe Greatests:

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CRYSTAL:

Ali was ready to spar another round, this time in a verbal war of words at breakfast with roommate Katharine Hepburn.

"If you put another biscuit in my face, I'm going to knock you upside your head ! I am still the greatest of all time!"

Ok, this wasn't the real-deal Ali but rather Crystal as he performed another spot-on impersonation of the former heavyweight champ (opposite Martin Short as Hepburn) in a 1980s "Saturday Night Live" skit.

Ali was occasional guest -- when he wasn't the subject of a roast -- at Crystal's touring shows.

"He's been a truly great friend to me and a great friend to the world," Crystal said when he introduced Ali before a performance.

Ali was a great sport when it came to his public persona, as well. Ali took the pokes and jabs with more ease than when he was in the ring from comedians like Crystal and various other TV shows and personalities who found Ali's larger than life aura easy to duplicate on stage or screen.

Crystal and Ali forged a friendship that spanned 40 years. But in the battle of the one-liners over the course of their friendship, Crystal was the undisputed champ of comic relief.

"I'm announcing tonight that I'm changing my name again because I've got new religious beliefs," Crystal said, as Ali, on a 1977 episode of "The Tonight Show." "From tonight on, I want to be known as Izzy Yiscowitz. I'm a Hasidic jew. Izzie Chaim Yiscowitz. That stands for Chaim the Greatest."

Crystal also roasted Ali during a guest spot for the fighter's 50th birthday celebration TV special.