Police were called at 2am on Thursday, 11 July to Harrow Manor Way following reports that a man appeared to have fallen from a flyover onto the carriageway.
Officers and the London Ambulance Service attended. The 33-year-old man was taken to a south London hospital where he remains in a critical condition.
Officers are in the process of tracing his next of kin.
Detectives are keen to trace the driver of a silver car that was in the area at the time. They would also like to speak with anyone else who may have witnessed the moments leading up to the man falling.
Anyone with information is asked to contact the South East Area Command via 101 quoting reference 703/11Jul.