Michel Platini has begun his appeal at the Court of Arbitration for Sport against a six-year ban by FIFA over a $2 million payment approved by Sepp Blatter.
The UEFA president did not speak with reporters Friday after arriving for an 8 a.m. closed-door hearing expected to last at least eight hours.
A verdict could come as early as Monday, when UEFA gathers in Budapest, Hungary, ahead of an annual congress of 54 soccer federations who have been without their leader for seven months.
"Hopefully it will be early next week, maybe a little later,'' CAS secretary-general Matthieu Reeb said. "The CAS tribunal will adapt itself to the needs of the parties.''
The three-member CAS panel is judging Platini's case afresh and has the authority to impose a life ban for corruption.
Previously, FIFA's ethics and appeals committees ruled out bribery as a factor and found Platini and Blatter guilty of charges including conflict of interest and disloyalty.
Blatter, the former FIFA president, arrived at 10:30 a.m. to serve as a witness. Blatter employed Platini as a presidential adviser from 1999-2002.
"I accepted this task. I'm on good form and I'm happy to be a witness in this matter,'' Blatter said outside the court.
Platini and his former mentor deny wrongdoing, and claim they had a verbal contract for the additional money. FIFA eventually paid Platini three months before Blatter was re-elected as president in 2011.
Both are effectively the star witness in each other's appeal case, after they were heard on back-to-back days by the two earlier FIFA tribunals.