Honda World Touring Car Championship driver Tiago Monteiro says he was knocked out in his testing crash at Barcelona last month, which was caused by brake failure.
Monteiro was involved in a crash at the Spanish Grand Prix venue's first corner on the last of three days of testing.
He was taken to a local hospital and kept in for observation, but was later transferred to hospital in his native Portugal.
The championship leader announced he would be returning home a fortnight after his accident and, having started his recuperation process, is expected to participate in next weekend's round in China.
No details about the incident had previously been revealed but, explaining the crash in a video for the WTCC's official website, Monteiro said his Honda Civic had gone backwards into the Turn 1 wall at the end of the main straight after his brakes failed.
"[It happened] on the last run of the last day, which is amazing," he said. "It was 10 to 6pm.
"At the end of the straight line, which is probably the hardest braking point of the racetrack, I had no brakes. We had a technical issue.
"It's a really strange feeling to have no brakes because your body is waiting for a deceleration and nothing happens.
"So it was not the ideal point for that to happen because you are at 255km/h [158mph], you brake very late.
"But still I tried to get away from the wall in front of me, I tried to escape through the right side [and] go to corner number two but unfortunately the car jumped on the grass and when it jumped on the grass it started to spin.
"I had a big impact backwards and then lateral, which was obviously quite hard, big Gs but the snap, the whiplash was the part that hurt me the most. All the right side of my body was bruised and hurt.
"I realised I was going to hit the wall, I protected myself but I don't remember the impact; although I do remember the Nordschleife impact very much [when Monteiro's tyre failed in 2016], it was very painful, this one was so hard that I got knocked out."
Monteiro said it was the first time in his career he had been "really hurt" in a crash.
"I've been racing for 20 years and I've had a few crashes, a few accidents," said Monteiro.
"Some actually quite hard in Champ Car, in Formula 1 I had a few hard ones.
"Even in WTCC, I've been there for 10 years now and we had a few hard crashes, especially last year in the Nordschleife.
"But this one after 20 years was the first time that I really got hurt.
"I must say I was really quite lucky in 20 years to never really get hurt but this one did hurt."