Charles Leclerc says he feared Romain Grosjean was dead following the Frenchman's fiery crash in Bahrain.
Grosjean spent three nights in the hospital before being discharged on Wednesday morning after receiving treatment for burns on both hands
Leclerc, speaking with BBC Sport, said: "I saw the crash in my mirrors, was aware straight away that it was extremely serious and I thought the worst straight away."
"Unfortunately, there was no news until I got back into the pits and got out of the car to finally know he was out, which I couldn't really believe, especially because coming into Turn Nine, I saw the flames and the fire."
Leclerc has dealt with plenty of personal tragedy. Jules Bianchi, who suffered fatal head injuries in a crash in the 2014 Japanese Grand Prix, was godfather.
Meanwhile, just last year, Antoine Hubert who was killed in a Formula 2 crash at last year's Belgian Grand Prix, was a childhood friend of Leclerc.
Leclerc said in the hour before the re-start in Bahrain on Sunday he tried to put the incident out of his mind by focusing on the race ahead.
"I just put myself in my bubble as I always do and tried to focus on my own thing," the 22-year-old said. "There are a lot of things you need to have in mind before an F1 race, which is the strategy, how you want to attack in the first laps and all these things.
"I was just focusing on those things to achieve the best result, trying to not think about the crash any more, mostly because I knew Romain was out of the car and fine to the extent he could walk by himself to the ambulance.
On Thursday, Grosjean returned to the paddock and is hoping to make a recovery in time to take part at the final round of the 2020 season in Abu Dhabi next week, which is also likely to be his last in F1.
Leclerc continued: "I sent him two messages. The first one, I don't think he saw it because probably he received thousands of messages and the second one he replied, probably because he could write again, as I have seen on social media that they took off his bandages.
"He was able to text again and I was happy and he seems to do fine. I didn't go to see him at the hospital because I expected lots of people there and I didn't want to bother him."