A Henfield man was taken to hospital with head and leg injuries after his car was involved in a collision with a tree on Sunday (November 9).
The incident happened at around 6pm on the B2116 Henfield Road, near Blackstone Gate Farm.
The driver was cut from the car by firefighters using hydraulic rescue equipment, and was taken to the Royal Sussex County Hospital in Brighton.
A Sussex Police spokesperson said the 23-year-old, who was driving a grey Renault Clio, was seriously injured.
The road was closed for more than four hours.
Police are investigating and are appealing for information.
Witnesses can email firstname.lastname@example.org or phone 101, quoting Operation Storey.