Police is appealing for witnesses after the 60-year-old man received the head injury at about 12.20am.
He was taken to Wexham Park Hospital, where he is being treated for serious head injuries.
Officers are keen to talk to anyone who saw the man before the incident, or who saw him fall over.
Detective Sergeant Peter Wall said: “This man received some very serious head injuries so we are working hard to establish exactly what happened.
“If anyone saw a man walking along Sunninghill Road at around midnight, or who witnessed someone fall over, please let us know.”
Anyone with information is asked to contact Det Sgt Wall on 101.