Growing up in a village near the city of Yangtai in Shandong province, Cai, an athletic girl who could not sit still, was noticed by the state sports system as a preteen.

She was offered a place in ti xiao (sports boarding school) in the nearby city of Zhaoyuan. Cai says that at the time she knew so little about the world that she was not even aware that boxing existed, although she loved watching martial arts films.

She immediately agreed to leave her family and move to the school. “Before letting me go, my mother asked me if I was afraid of hard work,” recalled Cai in a documentary. “I said, ‘No’.”

Cai started with freestyle wrestling but then switched to sanda – Chinese kick-boxing – because she liked “combat that uses striking”.

Not every child can handle being forged into an athlete and Cai reached her breaking point when she was 17. Battling to cut weight before a sanda tournament, she had to starve herself and was running in the heat wearing several layers of clothes. “I suddenly sat down on the running track, crying, and decided I was going home,” she said in a television interview.

She had packed up her things when it dawned on her: “What will I do [to make a living] when I get home?”

Cai unpacked her bags. She did not want to be seen as a failure by her family – leaving home had been her decision. She eventually became provincial sanda champion.

After sanda, Cai tried Muay Thai and then took up boxing. At 23, she joined China’s first professional boxing gym in Kunming, training alongside future world champion Xiong Chaozhong.

Cai told the Post that her motivation to become a professional boxer was not money. She “wanted to achieve”. She was working as a sanda coach in the mornings and trained in the afternoons.

“Cai Zongju’s technique is excellent. She hit me with a few shots when we were sparring. Her speed and power, everything is at very high level – she improved very fast,” Xiong told the Post.

Cai made her professional debut in September 2014 in Kunming, winning a decision against compatriot Li Yunting. Five weeks later she fought again, in Laos, losing on points to home fighter Myra Ketsana. “A little unfair,” was Cai’s verdict of the judges’ decision.

Cai won the WBC International minimumweight belt against Filipino four-time world title challenger Gretchen Albaniel in her fifth fight. She then defended the title three times.

She then claimed the IBF intercontinental minimumweight title, outpointing Thailand’s Siriporn Tawesook, a former world champion with an outstanding record of 37 wins and three losses.

In January 2017 in Macau, Cai faced Etsuko Tada for the IBF world minimumweight title. Japan’s Tada had been a unified WBC and WBA champion. Cai won on a split decision and became world champion in her 10th fight.

She made one defence against old foe Albaniel, but then gave up the belt. In June 2018, in her last professional fight, Cai claimed the vacant fringe IBO world championship against Mexico’s Ana Victoria Polo.

Cai’s record now stands at 11 wins and one defeat, earned against quality fighters – the combined record of her opponents stands at 123 wins and 41 losses.

Cai, who also beat three former world champions, switched to amateur boxing in 2018, hoping to make the Tokyo Olympics. She immediately made the provincial and then the national team, representing China at the amateur world championships in Russia. She was eliminated at the quarter-final stage and says that she has now returned to professional boxing.

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Cai is an elusive, skilful counter-puncher, not a knockout artist – all but one of her professional victories were won on points.

“Boxing is intricate. It feels like an art – a punch you throw can be beautiful, slipping a punch can be beautiful, a counter can be beautiful, the moment of knockout is beautiful. Like dancing,” Cai said in a documentary.

“I can fight on the attack,” Cai told the Post. “But why should you go all out aggressively if there is a better way to win?”

Despite her stand-out defensive skills, Cai’s sanda coach Chen Yusheng says Cai’s biggest asset is “kang ya” – the ability to deal with pressure.

Cai had to call up all her kang ya in the second defence of her WBC International belt, against Japanese former two-weight world champion Mari Ando. Two weeks before the fight, Cai had her nose broken in sparring, but assured her coach that she could still fight.

For all 10 rounds Ando charged forward, while Cai danced out of the way, landing crisp, hurtful counters. Whenever Ando had her pinned, Cai refused to be bullied, trading power shots in vicious toe-to-toe exchanges. She squeezed out a majority decision win.

“My white shorts were completely red with blood and I came close to being TKO’d. I took a lot of punishment. This was my hardest, but also my happiest fight – I improved in every way,” Cai said.

Despite the success and popularity of female fighters, such as UFC champion Zhang Weili, gender stereotyping is still prevalent in China. In one television appearance, the female interviewer failed to comprehend why Cai chose to be a boxer: “But you are a girl! You are mommy’s treasure!”

Cai shrugs off these stereotypes – she sees herself as just another person who left the countryside and is fighting their way up without any privileges or connections. “Everyone has it hard. I just try the best I can. The world is never fair,” she said.

Then she added: “But perhaps I have seen more than most people my age.”