Sport's highest tribunal began a long-delayed, four-day hearing on Monday which could result in the three-times Tour de France champion Alberto Contador being stripped of his 2010 title for doping.
Contador tested positive for the banned anabolic agent clenbuterol during the 2010 Tour and was subsequently cleared of any wrongdoing by the Spanish cycling federation (RFEC) which accepted his explanation that he had eaten contaminated meat.
The International Cycling Union (UCI) and the World Anti-Doping Agency (Wada) both appealed to the court of arbitration for sport (Cas) over the Spanish ruling, demanding that the rider should be banned.
Contador did not comment as he arrived at the Cas on Monday for the hearing, postponed twice from June and August, with Spanish officials, and will not speak to the media throughout the duration of the hearing, his representative said.
A verdict is not due to be announced until early next year.
The Israeli lawyer Efraim Barak will preside over the three-man arbitration panel. Switzerland's Quentin Byrne-Sutton was nominated to the panel by the UCI and Wada while the German Ulrich Haas was nominated by Contador and RFEC.
Some 23 people are set to appear as witnesses or experts at the hearing with, on Contador's side, the American lie detector expert Louis Rovner reportedly using polygraph analysis of a statement by the rider.
Contador, 28, won the Giro this year after he resumed competition following the RFEC's decision to clear him of wrongdoing.
If found guilty, the Spanish rider would be stripped of his 2010 Tour title and would also be likely to lose his Giro title under the UCI's anti-doping regulations.
Any suspension would start, according to UCI rules, "on the date of the hearing decision", but Contador would be credited for the four and a half months of his provisional suspension which ended on 15 February.
The hearing could also turn the spotlight on the recent decision to clear 109 footballers who tested positive for clenbuterol at the World Under-17 Championships in Mexico this year.
Jiri Dvorak, head of football's world governing body Fifa's medical services, said the players were not punished because their cases were not considered as doping but rather as part of a big health problem in Mexico.
This decision was accepted by Wada which also agreed not to ban five senior Mexican players who tested positive for the substance at the Concacaf Gold Cup this year.
Mexican authorities have admitted the country has been affected by the practice of injecting cattle with the steroid, which is banned by Wada.