Nafi Thiam pushed to last drop of sweat by Katarina Johnson-Thompson

Nafi Thiam looks worried. Katarina Johnson-Thompson looks worried. It’s a little after 8.30pm and the final race of the heptathlon is about to start, but really these are largely incidental details. There is a pretty good chance that at any randomly chosen moment in their week, Thiam and Johnson-Thompson can be found mid‑frown, wearing the grave and harried expression of someone constantly fretting that they’ve left the front door unlocked.

Perhaps this is simply the heptathlete’s condition. There are never enough training hours in the day. There is always something that needs fixing, some injury niggle that won’t go away. The heptathlete cannot strut their way around the arena, strike a pose for the cameras, ease down towards the end of their heat. Every second, every fraction of that second, every last joule of energy in their body, needs to be directed towards those seven gruelling events, and the margins that decide them.

Related: Team GB’s women take thrilling relay silver before men hold on for bronze

And so perhaps it is no surprise that they typically exist in a process of near-constant anxiety, never quite feeling that they’ve done enough. Three years ago, when Thiam won her second Olympic gold in Tokyo , she crumpled over the line in the 800m and just burst into tears: not happiness but purgation, those long months of agony and stress finally at an end.

Thiam turns 30 this month, but this is an event in which you measure your age not in years but in scars. The great Carolina Klüft retired at 29, Jessica Ennis‑Hill at 30. Denise Lewis hung on until 32, and regretted not going sooner. “Every part of my body has suffered at some point in my career,” Thiam said ahead of these championships. “Achilles tendons, back, shoulders, elbows.”

Now she has hinted that these may be her last Games, and this latest cycle has also been defined by interruptions and upheaval. She left her veteran coach Roger Lespagnard, who took her on when she was just 14. She missed the world championships in Budapest last year, which Johnson-Thompson won . And by a weird quirk of the treatment table, the world’s two best heptathletes have never gone head to head in a major championship where both were at their best. Until now.

Over the last two days, a genuine classic has been brewing. Their strengths and weaknesses are familiar to us all now. Johnson-Thompson takes a small lead after the sprint hurdles. Honours even in the high jump. Big personal bests for both in the shot put. Through the 200m and the long jump and the javelin, which all go pretty much as expected, the competition slowly winnows away until finally, Thiam and Johnson-Thompson are staring each other down.

You may consider Johnson-Thompson unfortunate to have existed in the era of one of the greatest heptathletes of all time. But that would be to understate the way these two women have pushed each other, forced each other to become more complete athletes. Thiam is a much better runner these days. Johnson-Thompson has worked heavily on her throws, but lost a little of her raw pace in the process. And so here they are, side by side, the track glistening with rain, Johnson-Thompson needing to find 8.5sec to win Olympic gold.

At times, it looks like she might do it. Thiam stumbles on the first lap but stays up. Johnson-Thompson paces herself behind the strong Anna Hall and slowly builds a lead. Round the last bend, she winds it up. Thiam, watching her rival disappear into the distance, can only scrap and suffer. She finishes six seconds back. Personal bests for both. In the final race of the Olympics. After that . It’s unfathomable.

You can torture yourself with the margins. Where could Johnson-Thompson have made up those 36 points? Three centimetres in the high jump. Twelve in the long jump. A quarter of a second in the 100m hurdles. Over two days, over four years, it’s nothing. Better by far to savour the journey, to take pleasure in the journey, in pushing what we now have to call one of the greatest female athletes in history to her very last drop of sweat.

Thiam is an utter phenomenon. Were she American, she would be one of the most famous athletes on the planet. She would have Taylor Swift selfies and battleships named after her and teens posting TikToks of themselves shouting: “SLAY! SLAY! SLAAAY!” Yet at the same time she is a natural introvert, suspicious of the spotlight, not overly comfortable selling herself with all the expectation and intrusion and extra work that entails, far happier with a hard day’s training and then a long rewarding evening with a Colson Whitehead novel.

I went to her house once for an interview and we ended up talking about geomorphology, how the earth is shaped by things such as rivers and glaciers. It was one of my favourite ever interviews. But I didn’t sense Saturday Night Live were going to be beating a path to her door any time soon.

So let her work speak for itself. After the end of the 800m, everyone collapses to the track, utterly drained by their efforts. Then, slowly, they peel themselves off the track, embrace and set off on a collective lap of honour, a recognition of their shared expedition. And as they circle the track, Thiam slots unassumingly back into the pack: first among equals, still the best‑kept secret in athletics. But if you know, you know.