The car would 'almost certainly' have killed a patient on any other day said Dr Huw Morris of Northlands Wood surgery in Haywards Heath.
The driver, thought to have blacked out at the wheel in the practice car park, escaped with cuts and bruises.
'The receptionists heard a sort of explosion, and came in to see this man trying to climb out of his car in my consulting room,' said Dr Morris.
At 11am on any other day, Dr Morris would have been seeing patients. The car smashed through a window and came to rest inches from the patient's chair, which was peppered with bricks and debris.