The Italian was airlifted to hospital on Friday following a crash with a truck that left him with serious head injuries.
Having undergone surgery, Zanardi is in an induced coma and on a ventilator, with his condition described as 'stable' by the Siena hospital that is treating him.
An official update from the Santa Maria alle Scotte hospital said: "The clinical conditions remain unchanged in the cardio-respiratory and metabolic parameters while the neurological picture remains serious.
"The patient is sedated, intubated and mechanically ventilated and the prognosis is confidential.
"The multidisciplinary team that takes care of him will evaluate in the next few days any diagnostic-therapeutic actions to be taken."