Met Police confirmed at 5:30pm on Monday they were called to Upper Green East in Mitcham, South West London.
But when they arrived at the scene there was no trace of a victim.
Police were later called to a hospital.
Currently no arrests have been made but a crime scene still remains in place.
A stop and search power has been authorised for the entire London Borough of Merton.
The injured man was said to be in a critical condition at the hospital.
In a statement, the Met Police said: “Police called at 17:30hrs to reports of an assault on Upper Green East in Mitcham.
“Officers attended but there was no trace of a victim.
“Called a short time later to a hospital after a man, aged early 30s, self-presented with stab injuries. Condition is critical.
“Crime scene is in place. At this early stage, there have been no arrests. Enquiries continue.”