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 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.