Officers fought to stop flames engulfing a Volkswagen Polo after it was in collision with a BMW in Hailsham on Sunday (September 1).
Three people were injured in the collision on the A267, north of the Boship Roundabout, near Hailsham, at about 3pm, police said.
Police said when officers arrived at the scene they used their own fire extinguishers and one from a nearby garage to contain the fire and then assisted in the emergency treatment of the occupants, who were trapped.
They were supported by another motorist.
The driver of the Polo, a 22-year-old man from Tunbridge Wells, sustained serious injuries and was taken by air ambulance to the Royal Sussex County Hospital in Brighton.
His passenger, a man aged 26 also from Tunbridge Wells, was taken to the Eastbourne District General Hospital with minor injuries, police said.
The BMW driver, a local man aged 25, is being treated at the Royal Sussex County Hospital for serious injuries, police said.
Sgt Stacey Ellott said: “The quick actions of the officers helped save the life of the driver who was trapped in the burning car. He and the other driver are in a serious condition and they are receiving treatment.
“I would like to thank motorists for the understanding as the road needed to be closed for nearly five hours while officers investigated.”
Police are appealing for anyone who witness the collision to call 101 quoting Operation Nova or email email@example.com.