City's Bernardo Silva and Mateo Kovacic both misfired in the shootout to put Madrid in the driving seat
The 35-year-old was on his way to celebrate his mother's birthday in Porto Alegre when the vehicle driven by his personal chauffeur veered of the road, Xinhua reported.
Nobody was injured in the accident and there were no other vehicles involved. Photos published on social media showed the car tilting in a shallow roadside ditch.
The accident came just days after the former Barcelona star left Brazilian outfit Fluminense by mutual consent.
He failed to score or provide an assist during his nine matches in three months for the Rio de Janeiro club.
Assis has denied the forward might retire and promised "there could be some surprises" amid speculation linking him to the US and China.