Around 9.10pm, a red Seat Ibiza was travelling south on the A7, approximately half a mile north of Galashiels when it collided with a black Seat Ibiza travelling north on the other side of the carriageway.
The 20-year-old driver of the red car was taken to hospital, where he is said to be in a critical condition.
The driver of the black car was examined by ambulance staff at the scene and did not require to attend hospital.
An investigation is under way to establish the full circumstances of the incident and the road was closed for several hours to allow crash scene investigators to examine the scene.
Sergeant Jill Kirkpatrick said: “Enquiries carried out so far have established that another black car, make currently unknown, was in front of the red Seat prior to the collision and I would ask the driver of that vehicle to contact us as they may have information which could assist our enquiries.
“I would appeal to anyone who was on the A7 around the time of the incident to contact us with any information or dash cam footage as it could prove vital in our investigation.”
Anyone with information is asked to call Police Scotland via 101, quoting incident number 3191 of Monday, December 27.