Olympic 800 metres champion Caster Semenya of South Africa went to the Court of Arbitration for Sport on Monday to challenge proposed rules that could force her to lower her testosterone levels.
Semenya made no comment as she arrived at the court in Lausanne for the start of a week-long hearing that is likely to define the rest of the 28-year-old's career.
The International Association of Athletics Federations (IAAF) says it is introducing the rules to create a "level playing field" for other female athletes.
The South African government says the rules specifically target Semenya and has called them a "gross violation" of her human rights.
The controversial measures would force so-called "hyperandrogenic" athletes or those with "differences of sexual development" (DSD) to take drugs to lower their testosterone levels below a prescribed amount if they wish to continue competing.
The rules were to have been introduced last November but have been put on hold pending this week's hearings. A judgement is expected by the end of March.
As he arrived at the court, IAAF President Sebastian Coe said: "Today is a very, very important day.
"The regulations that we are introducing are there to protect the sanctity of fair and open competition."
Athletics South Africa has strongly backed Semenya. Its chief advocate Norman Arendse said she would give evidence.
"The whole week is going to be important. Obviously the evidence will be evaluated and assessed at the end of the process this week. so today this is the start," Arendse told reporters.
'INCREASES IN STRENGTH'
The issue is highly emotive.
When British newspaper The Times reported last week that the IAAF would argue that Semenya should be classified as a biological male -- a claim later denied by the IAAF -- she hit back, saying she was "unquestionably a woman".
In response to the report, the IAAF -- stressing it was referring in general terms, not to Semenya in particular -- denied it intended to classify any DSD athlete as male.
But in a statement, it added: "If a DSD athlete has testes and male levels of testosterone, they get the same increases in bone and muscle size and strength and increases in haemoglobin that a male gets when they go through puberty, which is what gives men such a performance advantage over women.
"Therefore, to preserve fair competition in the female category, it is necessary to require DSD athletes to reduce their testosterone down to female levels before they compete at international level."
Semenya is not the only athlete potentially affected -- the silver and bronze medallists in the Rio Olympics 800m, Francine Niyonsaba of Burundi and Kenya's Margaret Wambui, have also faced questions about their testosterone levels.
But it is Semenya, who also won Olympic gold in 2012 and has three world titles to her name, who has brought the court challenge.
Matthieu Reeb, the secretary general of the court, said: "It is unusual and unprecedented because we never had a such a case at CAS."
South Africa's Sports Minister Tokozile Xasa argues that the IAAF's rules are "discriminatory".
"What's at stake here is far more than the right to participate in a sport. Women's bodies, their wellbeing, their ability to earn a livelihood, their very identity, their privacy and sense of safety and belonging in the world, are being questioned," Xasa said on Friday.
On Sunday, tennis great Martina Navratilova threw her weight behind Semenya, saying it was significant that the rules would only apply to female athletes competing in distances from 400m to a mile.
"Leaving out sprints and longer distances seems to me to be a clear case of discrimination by targeting Semenya," Navratilova wrote in Britain's Sunday Times newspaper.
"And can it be right to order athletes to take medication? What if the long-term effects proved harmful?... I hope she wins."