On this day, March 31, 1973: Muhammad Ali suffers shock defeat to Ken Norton

The Louisville Lip is beaten for second time as first of razor-tight trilogy of fights takes place in San Diego

When Ken Norton stunned the boxing world by defeating Muhammad Ali in 1973, it would start in motion one of the sport's all-time great rivalries.

Discussion about Ali's big fights usually revolves around the likes of Sonny Liston, Joe Frazier and George Foreman.

Thrillas in Manilla and Rumbles in the Jungle always loom large but his three battles with the tough and tricky operator from Jacksonville, Illinois, should never be forgotten.

The heavyweight trilogy between the American pair would go on for 39 draining rounds over a three-year period, with Ali taking the honours by razor-thin margins, two wins and one defeat. Norton would insist he won all three.

The opening bout took place in San Diego, California, in front of 12,000 fans, two years after Ali had lost for the first time against Frazier in the 'Fight of the Century'.

It was supposed to be a routine night for Ali – with one defeat in 41 fights – who was a clear favourite against the unfancied Norton –  one loss in 29 – but would come up against an tricky, unorthodox opponent.

Ali would have his jaw broken after two rounds, yet the fight would still go the distance. "Imagine you had your jaw broken and have to fight 10 more rounds," Ali said afterwards

At the half-way point, a shock was clearly on the cards, with ringside commentator Howard Cosell saying: "Either Ken Norton is smarter, much better fighter than anybody thought, or Muhammad Ali has gone back a lot more than one would have reasonably believed." It was the former that was fairer and more accurate.

Norton took a tight split decision from the judges, throwing a spanner in the works of Ali's plans to take on Foreman, who had taken the world title off Frazier two months earlier.

The pair were back in the ring within six months. This time at the Forum, in Inglewood, California, it was Ali who took the split decision. But he admitted after the fight: "Ken Norton is the best man I have ever fought."

It would be another three years before the third an final bout, in the grand setting of Yankee Stadium in New York.

Ali's legacy as 'The Greatest' had been secured in the intervening years after stunning, career-defining wins over Foreman and Frazier.

The Louisville Lip was in full flow before the fight, as the WBA and WBC world champion said that he would "knock the sucker out inside five rounds". But Norton's style had always presented a problem that Ali had struggled to solve.

"Ali and his people tried to call me awkward," said Norton. "It makes me sound like I didn't know what I was doing. I was more than that and I just had the style, with the jab, to beat Ali. It's that simple."

And so it went the distance again and the judges gave Ali a unanimous, if hotly contested, victory. "I won at least nine or ten rounds. I was robbed," said a furious Norton.

During an interview a few weeks after the fight, Ali admitted that he thought he thought he had lost that night in the Bronx. "Kenny’s style is too difficult for me. I can’t beat him, and I sure don’t want to fight him again," he said.

It might have been two frustrating defeats in a row for Norton, but it was that night in San Diego in 1973 that stunned the world and set him up for life, in and out of the ring.

“The first Ali fight gave me a chance to give my son more food, better clothes," he would later say. "A fight with Ali gave me a chance at life, period.”