The victim, aged 62, was discovered suffering from serious head injuries just after 5.15pm on Friday on Gleneldon Road.
Witnesses who saw the incident described how the man was punched after a "right-of-way dispute" and saw someone fleeing from the scene.
In a fresh appeal for information, police have described how the victim and another motorist exchanged words after both trying to drive through the narrow road and becoming stuck.
During the altercation the suspect reached out of their own car and through the victim’s window, before punching him in the face.
Both men then got out of their vehicles and the suspect punched the victim in the face again with such force that he fell to the ground and hit his head on the concrete.
The suspect then got back into his car and drove off down
Shafaq Hassan, a speech and language therapist who lives nearby, said: “We heard people outside our house and went to investigate.
“Passersby and neighbours had stopped. There was a man sitting on the road surrounded by a few people. I asked what had happened and was told he had been punched.
“As I'm an SLT I automatically sat down next to him to try and comfort him. He looked in shock and disorientated.
“He had a head injury on back of his head as apparently he hit the pavement and possibly passed out for few minutes."
She added: "I stayed with him as police and medics came. They asked questions to check if he remembered date and year. Eventually they put him on a stretcher and took into the ambulance.
“Other neighbours reported seeing him have a dispute with the other driver about who should give way at the junction."
The victim was rushed to hospital where he remains in a serious but stable condition.
Detective Constable Caroline O’Shaughnessy, from Brixton CID, said: “This was a nasty assault, prompted by what appears to be road rage, carried out in broad daylight at a time when there would have been plenty of people making their way home.
"The victim suffered a serious head injury as a result of being punched to the ground following a verbal altercation, and this assault could easily have had more grave consequences.
“If you saw what happened or you have information that could assist us, we would urge you to get in touch.”
The suspect is described as a black man, aged between 20 and 25 years old and around 5ft 8ins tall with afro hair. He was wearing a jumper and black trousers and was driving a dark green-coloured car.
Anyone with information is asked to call 020 8649 2156 or tweet @MetCC. Alternatively, call Crimestoppers anonymously 0800 555 111.