After two days of exhilarating competition in the world’s most prestigious heptathlon event, Katarina Johnson-Thompson tumbled across the line in the 800m before dropping like a swatted fly. She then lay as still as the dead for several minutes, and when she finally moved it was only to throw up. But it was worth it. For while she had obliterated her body, she had also destroyed her personal best and a world-class field by 337 points.

It seems scarcely believable that on Friday Johnson-Thompson was talking about suffering from impostor syndrome, a feeling of never being good enough or feeling like a fraud. Fraud? Not on this evidence. And not when her final score of 6,813 points would have been good enough to win gold in four of the past six Olympics.

“I was very sick after the 800m,” she said, smiling. “I didn’t think I would get to the line. I thought the stretcher would have to come and get me. But I’ve been wanting to get 6,800 for so long now so I’m so, so, happy.”

The only regret was that the current Olympic and world champion Nafi Thiam was missing here, because it surely would have made for a battle royale between the two best heptathletes on the planet. That will come though, in Doha this autumn and then at the Tokyo Olympics next year, and Johnson-Thompson believes she will be ready. “I’ve not always put it all together and I still haven’t this time, so it just gives me so much confidence going forward,” she said. “While this was a personal best, I can still see room to improve.”

The 26-year-old had gone into day two on 4,034 points – 177 clear of her rivals – but her morning started badly when she fouled her first attempt at long jump. In the past she might have allowed doubts and demons to come flooding back. Instead she took a deep breath, went back to the runway and jumped 6.68m into a 0.2m headwind – her joint best in a heptathlon.

Next up was the javelin, an event where she usually haemorrhages points. Not this time as she threw 42.92m – a massive personal best by 76cm – much to her delight.

That left her needing to run 2:09.19 in the 800m to shatter the 6,800 barrier, and she certainly was not going to die wondering as she blasted through halfway in 59.41 – a respectable time for the world’s elite, let alone someone in the final event of a two-day heptathlon. Inevitably she crashed in the last 400m but her time of 2:08.28 left her with plenty in hand to beat her personal best and make an emphatic statement of intent.

For years she has been regarded as flaky, someone who could not be trusted to win on the big stage. That was not entirely fair given she was on course for gold at the 2015 world championships only to foul three times at the long jump, the final attempt by millimetres, and was hiding an injury when she finished sixth at the Rio Olympics. But it stuck.

In the past 15 months, though, she has won world and European indoor titles, and a Commonwealth gold medal. She is certainly more robust now – although, as she admitted afterwards, she cannot always escape feeling like an impostor.

“Now and again I still get glimpses, for example in the long jump when I fouled the first attempt,” she said. “I still switch back to 2015 and then I have to switch out of it. It is definitely still in me but thankfully I can control my moods and focus on what I need. I have to thank my coach Bertrand Valcin and all of my training partners just pumping confidence into me. I can definitely see how I can switch out of that mentality and hopefully do more of what you saw.”

Johnson-Thompson certainly has no regrets about a move to Montpellier at the start of 2017 to train with the decathlon world record holder Kevin Mayer and his training group. At the time it was regarded as the track and field equivalent of shock therapy, given she had little grasp of the French language or culture, but the difference since has been obvious.

Not only have the niggles and injury pangs that interrupted her early career cleared up but her mentality is noticeably stronger too. As Valcin put it afterwards: “The most impressive thing about her now is that she always finds a way out.”

Valcin puts that down to working on her confidence as much as her technique. As he explained: “Our priority is never to push Kat too much in training. We want a good balance for the body. But we have to push her to trust herself. That is very important for her to win in Tokyo 2020. And she finished with a big performance so it was a perfect weekend.”

Well, not quite perfect. When asked how her French was now he began to smile. “No, catastrophe!” he replied. “But it’s good for me because I have to improve my English.” And when asked what she could do next he smiled. “There are no limits,” he said. On this evidence, you can believe it.