On March 24, 1975 World Heavyweight Champion Muhammad Ali was taken the distance in a 15-round TKO of challenger Chuck Wepner. Wepner came into the fight with a record 30-9-2. 

The bout was at the Richfield Coliseum in Richfield, Ohio. In round 9 of the fight, Wepner landed a devastating body blow to Ali that put the champ down on the mat. This was only the 3rd time in Ali's illustrious career that he went down for a hard count.

Ali claimed that Wepner stepped on his right foot and after some controversy, video seems to have confirmed Ali's claim. After the knockdown, Ali was visibly shaken but also extremely angry. For the final 5 1/2 rounds, Ali was himself as he made Wepner pay with a flurry of jabs. He finally knocked out Wepner with 19 seconds remaining in the 15th round. However, to Wepner's credit, Wepner beat the hard count at 7 and was back on his feet but the referee stopped the fight at that point. 

"Sylvester Stallone watched Wepner's fight against Ali and shortly afterwards wrote the script for Rocky, but Stallone subsequently denied that Wepner provided any inspiration for the movie. Wepner filed a lawsuit which was eventually settled with Stallone for an undisclosed amount.

It is also speculated that the 1982 film Rocky III was influenced by his fight against Andre the Giant, as the movie features a match versus wrestler Hulk Hogan as "Thunderlips", who throws Rocky out of the ring."

Link: https://en.wikipedia.org/wiki/Chuck_Wepner