Sport's highest tribunal began a four-day hearing, on Monday, which could result in 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, 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.
A verdict is not due to be announced until early next year.
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 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.
