On the far left is Australian athlete Madeleine Pape. She will run badly in this heat of the 800 metres at the World Championships in Berlin in August 2009, finishing second-last. In the middle of the pack is Caster Semenya, who is just about to become the most divisive athlete of the year.

Minutes before Semenya races in the final, the news will break that the International Association of Athletics Federations has asked her to take a gender test. Despite the furore, Semenya will blitz  this race too, winning the 800 metres by two-and-a-half seconds, the biggest margin in World Championship history, and bounding into the middle of a furious debate about sex testing in sport.

As Semenya was hustled back to her rooms by South African officials after the race, Pape was nursing her wounded pride on another floor of the same hotel.

After that heat, she had told the Australian press she was "really pissed off" with herself for being distracted by a competitor's fall and losing her composure on the second lap. But there was something else she didn't say.

She was also really pissed off about Semenya. Had she been beaten by someone with an unfair advantage? Someone that a spokesman for the IAAF said was "a woman but maybe not 100 per cent". People like Caster, she felt, shouldn't be allowed to compete.

It would take a professional catastrophe and a personal transformation to change Pape's perspective.

In 2010, Pape badly damaged her Achilles tendon, She tried to run through the pain and the injury to qualify for the Commonwealth Games team, but "ended up having to have a couple of surgeries, and not really ever being able run again".

"I didn't have much choice in the end," she says – she had to reinvent herself.

She moved to the US to study sociology at the University of Wisconsin-Madison. On campus, she found herself debating these issues with people who thought entirely differently about gender as something fluid, not fixed or binary. She started questioning her previous faith in the science that underpinned sex testing in sport.

"It was really confronting, to be honest," she says now.  "Athletes trust that governing bodies like the IOC are making decisions based on the best available science."

She no longer believes that to be the case.

Athlete-turned-academic Madeleine Pape. Photo: Supplied

Seven years later, Pape has become a supporter of Semenya, who is in career-best form. The South African athlete won the 800 metres on Friday at the Monaco Diamond League in a personal best time to underline her status as a heavy favourite for next month's Rio Olympics.

Pape has also become a vocal critic of the IOC's sex testing of female athletes. Last year, she testified in support of Indian sprinter Dutee Chand, another athlete who made international headlines when she was dropped from the 2014 Commonwealth Games squad at the last minute after the Athletic Federation of India ruled her testosterone levels made her ineligible to race as a woman.

Chand and her supporters challenged the ruling at the Court of Arbitration for Sport.

The outcry in 2010 after Caster Semenya was suspended from racing for two years had swung the spotlight onto sex testing in sport. A recent investigation in The New York Times called it a "humiliating practice" and a breach of human rights.

At Chand's appeal, Pape countered English marathon champion Paula Radcliffe's argument that elevated testosterone levels "make the competition unequal in a way greater than simple natural talent and dedication".

Testifying via Skype, Pape said there was no such thing as a "level playing field" – athletes competed with many advantages, from superior aerobic capacity through to access to the best training facilities.

Identifying herself as a "passionate advocate of gender equality in sport", Pape argued that the "hyperandrogenism" regulations used to disqualify female athletes were based on flawed science and upheld "dominant gender stereotypes".

In a historic decision, Chand's appeal was upheld. Sport's highest court ruled that women with naturally high levels of testosterone could not be barred from competing, and suspended for two years the "hyperandrogenism regulations" set by the International Association of Athletics Federation – long enough to give both Chand and Semenya a chance to compete at Rio.

Looking back now, Pape says that in 2009 she was struggling with "sour grapes or tall poppy syndrome".

"I think it's easy to look for ways to discredit the performances of people who have beaten you even when they've done it fairly and squarely, especially when you haven't performed as well as you were supposed to."