Kevin-Prince Boateng and AC Milan coach Massimiliano Allegri are testifying in the racism trial against six fans of lower division club Pro Patria.
After repeated racist chants were directed his way during an exhibition game in January, Boateng kicked the ball in anger at a section of the crowd, took off his shirt and walked off the field with his Milan teammates.
The fans face charges of inciting racial hatred.
Milan players Urby Emanuelson, Sulley Muntari and M'Baye Niang were also targeted by the chants.
Milan defender Daniele Bonera was also testifying Wednesday.