There was no surer thing at London 2017 than Caster Semenya winning 800m gold. The unknown was how fast she would go. The South African defended her title in a stunning personal best of 1:55.16, the eighth fastest of all time. But considering she breezed over the line after a blistering 27.8sec final 200m the mind boggles at how fast she is capable of going. Surely Jarmila Kratochvilova’s world record of 1:53.28 is within her reach?

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Whether the world ever gets to find out will probably be decided not on the training track in Pretoria, where she is based, but in the picturesque Swiss town of Lausanne where the court of arbitration for sport is based because Semenya’s story is not as straightforward as her victory was last night.

Even while these Championships were part way through, Seb Coe was reiterating the intention of the IAAF, of which he is president, to implement a rule which would force the 26-year-old to have hormone replacement therapy or quit the sport.

Lord Coe confirmed that the IAAF will return to CAS this autumn to continue arguments over the case of the Indian sprinter Dutee Chand, who accused the organisation of discriminating against her by setting an upper limit for the testosterone levels of female competitors. The outcome of that case will have a direct effect on Semenya, who has been here before.

After she won 800m gold at the world championships in Berlin in 2009, her first of three world titles, the IAAF determined a testosterone threshold which, if exceeded by a female athlete, would render her unable to compete in a women’s race. This meant Semenya could run again but only if she took medicine to suppress her testosterone. She won silver in 2011 and again at London 2012. But in 2015 the rule was overturned and the South African has dominated since, winning a 20th consecutive 800m race to the cheers of the crowd inside the London stadium.

Semenya was fourth through the bell and Francine Niyonsaba, of Burundi, and the USA’s Ajee Wilson tried to break her between 400m and 600m but Semenya was with them round the top bend and really turned on the burners with 100m to go. Niyonsaba held on for silver in 1:55.92 with Ajee Wilson of the USA taking bronze in 1:56.65. Afterwards Semenya paid tribute to the London crowd. “I just love you guys. It feels like home in London. I used to say Berlin was like my home. Obviously it is South Africa, but there are such fantastic people here. Beautiful,” she said.

“I’m lucky to have a great support team who work with me. Full credit to them. Another world title is a fantastic honour for me and I love to do it here in London. The crowd are so welcoming to me and it makes it feel even more special.”

On attempting an 800m world record next, she added: “We need to clear 1:55 first and it will require a lot of hard training. I have Olympic, world and Commonwealth titles now so maybe it is time to target the world record. It’s the next thing on the list. I know it will be difficult but I will have to attempt soon, maybe.”

Britain’s Lynsey Sharp knew it would take the race of her life to get into the medals and it did not materialise as she finished 8th in 1:58.98 “Obviously that is the worst result I could have had,” she said, “I committed from the first lap, I didn’t just want to go through the motions. But I didn’t have the legs with 250m to go. It’s been hard to get to this point, so I’m proud of that.”

It was Semenya’s second podium finish of the week after she pipped Britain’s Laura Muir to bronze in the 1500m final and reignited the debate about hyperandrogenism, the condition she has which is characterised by very high levels of naturally occurring testosterone.

Shamefully, she has been the subject of mean-spirited commentary – notably from Katie Hopkins, who even mocked the South African’s choice to wear a suit when she married her partner earlier this year.

Coe admitted he understood the sensitivities around Semenya’s case earlier this week but defended the IAAF’s quest by claiming he was doing the right thing by his sport. “This is an incredibly sensitive subject,” he said, “We are all fathers and brothers. I don’t want athletes being demonised but it is the responsibility of the federation to create a level playing field in female sport.

“We don’t want to turn this into a witch hunt. Nobody is choosing to cheat here. This is not an anti-doping issue – it is a biological one. I have to protect the sport, and of course we have done this carefully.”

Understandably feelings are strongest among those who compete in Semenya’s event, particularly because Niyonsaba and the Kenyan Margaret Wambui, who have been the dominant two-lap trio over the last couple of years, are also thought to have the condition.

Ajee Wilson’s agent, Ray Flynn, said earlier this year: “It’s an open secret that her event has become monopolised by athletes born with hyperandrogenism. It’s a difficult situation but ironically, they discriminate against women themselves. It’s not an even playing field.”