Lawyers for South African runner Caster Semenya have begun presenting her landmark case against the International Association of Athletics Federations (IAAF) in Switzerland.
The double Olympic champion at 800m is challenging new rules proposed by the IAAF, the governing body for world athletics, which would enforce limits to testosterone levels in female athletes.
Semenya has a condition called hyperandrogensim, which means she has naturally elevated levels of testosterone, and argues the rules are a violation of her human rights.
The five-day appeal at the Court of Arbitration for Sport (CAS) is one of the longest in the court's history and could have enormous consequences for Semenya and other female athletes.
Semenya took no questions when she arrived at court in Lausanne, but did form a victory-sign with her fingers and smiled. Earlier she tweeted "God wouldn't put you in it if He didn't prepare you for it" in an apparent reference to the case.
Semenya's lawyers earned a small victory as she was granted permission to release details of the experts testifying in her favour despite the court's previous decision to hear the case under strict confidentiality agreements.
They were granted the right to publish their witness list after the IAAF has earlier released a statement disclosing the expert witnesses it would call. The court reiterated that the hearing would be confidential and no more information should be released publicly.
In detail: the IAAF gender proposals that have divided sport
The IAAF's proposed rules would affect female athletes with naturally high testosterone levels running in middle distance track events between 400m and a mile.
The athletes would be required to take medication to lower their testosterone levels and would be unable to compete for at least six months if they wanted to carry on racing against other women. Otherwise they would have to race against men.
The IAAF argues high levels of testosterone provide a performance advantage for hyperandrogenous athletes over other women and is therefore aiming to preserve fair competition.
The IAAF planned to bring in the new rules in November but delayed their implementation until March after Semenya's case has been heard.
IAAF president Sebastian Coe also attended the first day of the hearing in Switzerland. He told reporters the IAAF's proposals aimed to provide a level playing field for female athletes.
"The core value for the IAAF is the empowerment of girls and women through athletics," he said. "The regulations that we are introducing are there to protect the sanctity of fair and open competition."
Semenya won gold medals in the 800m at the Olympic Games in London in 2012 and in Rio de Janeiro in 2016.
She underwent gender verification testing at the age of 18 and was out of competition for nearly a year, although the results of the tests were never made public.
If she loses her case she is likely to miss the World Championships in Doha in September and her future Olympic participation could also be in doubt with the IOC likely to follow the IAAF's course in bringing in similar regulations.
Last week the South African government called on the world to fight for Semenya against what it called a "gross violation of human rights law".
A verdict in the case could take up to a month to be reached, according to CAS.