Ali fought for two decades and racked up 56 wins against just 5 losses. He faced many tough fighters, including Joe Frazier in three epic battles. Their first fight was called the 'Fight of the Century', and their last was the famous 'Thrilla in Manila'. Ali also took on George Foreman in the 'Rumble in the Jungle', which many say is the biggest boxing event ever.
But it's Sonny Liston who Ali says terrified him. Ali fought Liston twice, first in February 1964 when he challenged for the world heavyweight title. Ali won that fight when Liston quit after six rounds, shocking everyone. They fought again in 1965, and Ali knocked Liston out in the first round.
Here's what Ali said about Liston:
"Sonny Liston was before I fought him considered one of the greatest fighters in history. He had a punch that knocked out Floyd Patterson in just about one round twice. He had one fist as big as both as mine. Everybody was scared of him. [I was] scared to death."
Liston's reputation was no joke. He knocked out Floyd Patterson in about one round. Twice. And his fists? Ali said one of Liston's was as big as both of his put together.
Ali went on to become a boxing legend, but Liston's career wasn't too shabby either. He won 50 fights and lost only 4 before he died at 40 in 1970.
You can hear Ali talk about how scary Liston was in his own words online. It's pretty surprising to hear such a great champion admit he was afraid, but it shows just how intimidating Liston must have been in the ring.