Last Saturday night in Manchester, Zak Chelli, a 28-year-old supply teacher from Fulham, produced one of the sporting upsets of the year when he knocked out Cuba’s esteemed David Morrell in a stoppage as shocking as it was compelling. Chelli had been offered the fight two weeks earlier and despite the limited preparation he proved himself a formidable late replacement.

Morrell was ahead on the scorecards, but he was hurt badly in the ninth round. In the 10th and last, Mr Chelli – as he is known to his pupils – delivered a blistering lesson before Morrell was rescued by the referee.

A joyous Chelli exclaimed he had just done what even David Benavidez could not manage. Last year, Morrell suffered the only previous loss of his career when the imposing Benavidez beat him on points. But Morrell had knocked down Benavidez, one of the most feared fighters around, to burnish his own reputation.

Yet, giving hope to supply teachers everywhere, Chelli beat Morrell in an uplifting boxing story that might have echoed Rocky, but for some key differences. In Hollywood’s take on boxing, Rocky Balboa steeled himself for his unlikely shot at glory by pounding slabs of meat in a slaughterhouse. Chelli, instead, was given a cheerful send-off by his class at Thomas Knyvett, a secondary school in Surrey.

“I said to the students: ‘I’m not going to be here next week because I’ll be in Manchester for fight week,’” Chelli says. “I said have a look at this guy, David Morrell, and they were like: ‘Sir, you’d better win or don’t come back, because it’ll be embarrassing for you.’ I was like: ‘I know.’”

Chelli looks briefly serious before he laughs at his own earlier naivety in a classroom. “My mistake when I first became a teacher was that I was always happy. Never smile when you’re a supply teacher because the kids will rip you a new one. They’ll take the mick out of you. If you’re a supply teacher, they think they can do what they want. So I’ve learned to always have a firm and steady look, be sure about yourself, don’t be happy and say this is a great day because that’s when they try and take advantage.”

He nods when I suggest this sounds like the fight mask a boxer has to use during the ritual face- off. “Before you get into the ring, you’ve got to mentally beat them and that’s how it is with students. You’ve got to let them know you’re going to be in charge.”

Chelli took charge of his brutal schooling of Morrell, but an insight into his intense challenge is sharpened by the fact that, this week, he is on holiday. He had no idea he would be fighting Morrell when, a month ago, he booked it for himself, his wife, Eliza, and their 18-month-old daughter in Tunisia – where his father and trainer, Zak Sr, was born. Their flights were the day after the fight.

The light-heavyweight had won 16 of his previous 20 professional bouts and also been the British and Commonwealth champion at super-middleweight. He was far from being a journeyman, but it seems incredible that he could beat a fighter as good as Morrell at two weeks’ notice while holding down a full-time job. The chance to test himself at world level came when Callum Smith, who was due to fight Morrell on 18 April, was injured. Chelli was asked if he was ready to step in at short notice and face Morrell on the undercard of the great heavyweight battle between Daniel Dubois and Fabio Wardley.

“I’m always training with my dad after work,” he says. “I’d train for four hours because my dad’s always said to me an opportunity will come. We don’t know who, when or where, but an opportunity will come because that’s how boxing works. You’ve got to be patient. When we found out Morrell was not going to be fighting Smith my dad went crazy on social media saying: ‘Fight my son, he’s the best.’

“We thought nothing of it until, on 27 April, I finally got a phone call saying the fight’s going to happen. So for two weeks I was fully focused for it and that’s when I made a video on my social media saying: ‘I’m going to knock him out, inshallah.’”

Chelli had real belief he could win because, since moving up to light-heavyweight, he has felt so much stronger. “In sparring I’ve been dropping people. I started believing in myself more, thinking that if I catch anyone with this right hand, he’ll go down.”

He was also driven by an almost desperate need to win. “I needed it, not for myself, but for my wife and daughter because for two years since the fight in Barnsley [when he lost to Callum Simpson], I didn’t make a penny from boxing. I said: ‘Even if I’m there with one arm or one leg I’m going to give it everything.’”

Chelli was trailing on the scorecards, but “from the beginning I noticed he wasn’t catching me with his jab. I was making him miss and he just went for the body. I was catching him with my right hand and I heard the commentator saying: ‘Why is he stepping back?’ He’s stepping back because I hurt him.

“I thought I was winning some rounds, but I understand this is pro boxing. He’s been a two-time world champion so if I don’t knock him out they might cheat me. I was creeping up on him and near the end my dad was telling me: ‘Unleash the right hand. Give him the pain.’ That’s what I did.”

He could not celebrate for long because “we drove straight back to London after the fight as I had a flight to catch. My dad drove for four hours and I watched the knockout. I was like: ‘Oh, goodness, I look scary.’”

Chelli also scrolled through “the thousands of messages I got. I’m still replying to them all because I feel obligated. It was crazy because I’ve had 1.2m views of my profile.”

He hopes to fight Smith next and eventually become a world champion, but insists he will remain a supply teacher. Chelli began teaching during Covid when his Italian mother persuaded him to find a proper job. He then resumed his teaching work in earnest after losing to Simpson.

“I go from school to school and take whatever lesson needs covering. I’ll teach from GCSE to A-levels because I was a pretty academic student myself. I got 20 GCSEs, because I wanted to cover as much as I could and then I went to university and got a degree in business management.”

Boxing and education have always defined him and his older brother, Yahia. “My dad’s been training us since we were kids. If you look at pictures you’ll find me at just a week old with boxing gloves. My dad used to be a professional boxer and me and my brother were both ABA [Amateur Boxing Association] champions. My brother was a GB champion as well, but when he turned 18 he decided to take an academic route and he’s now completed his PhD in mechanical engineering.”

Chelli loves teaching, but are his students always receptive? “Usually, there’s one in the class who tries to mess you up and bring the others with them. But I take that student out and speak to them. They usually have an issue at home or they don’t understand the work and probably need one-to-one assistance. Once they see you can help them it changes. There have been times when I’ve had to bring out the deep voice to boys: ‘Sit down, bags off the table or I’m calling the principal.’ But usually it’s calm.

“I also teach boxing to kids that have behaviour or special educational needs. It builds their confidence and communication skills. You need a lot of patience, but most of them are reaching their targets and enjoying it. It’s rewarding for me, seeing that I can change lives for good.”

He adds that “my wife works for Chelsea Football Club as part of the marketing team and now we’ve got a daughter, we’ve found a way. She’s lucky to work from home three days of the week and I can be a supply teacher three days a week and the other two days I’ll just teach the boxing. And every night I’m training.”

For Chelli, “the only issue is once the kids find out who I am then the whole school knows. Luckily, as a supply teacher, I change schools. I did work for one school for a whole term and whenever I walked down the corridors all the students would jump up and say: ‘Hi, Mr Chelli.’ Or you’d hear them whispering: ‘He’s the boxer.’ So it gets a bit chaotic.”

It almost sounds as if he would prefer a life of anonymity. “Yes. I’ll say my name is Mr Jones and not tell them I’m a boxer because it can get crazy if they find out who you are.”

Mr Chelli will return to his temporary post at Thomas Knyvett next Thursday and there will be no chance of hiding his real identity or sudden burst of fame. “I just hope it’s not going to be hectic, with too much shouting,” he says with a helpless grin. “If it’s chilled and I can teach then I’ll be happy.”