Alex Albon's return in Singapore three weeks after a medical emergency is "audacious", says George Russell.
Williams driver Albon suffered respiratory failure after an appendectomy and was on a ventilator in intensive care for nearly 24 hours.
"It's definitely audacious to come back for the toughest race of the season having only just recovered," said Mercedes' Russell.
"But it just goes to show the sort of grit and determination he has."
Albon said on Thursday he was returning earlier than he initially thought possible, and that it would not be until after practice on Friday at Marina Bay that he would know for sure whether he can race this weekend.
"It's quite a tricky one because you are basically waiting for your lungs to recover and at the same time your body cannot move as well as it normally can," said the 26-year-old.
"You can't just jump back into normal training. You have to slowly build into it.
"It was kind of starting Monday last week when we really started to push it and see what we can do. I treated it like a nine-to-five job, training and recovery. Recovery is really important.
"So basically throwing everything at it, and day by day it was getting better and better and we got to a point where the recovery was going really well.
"I don't think we truthfully had in mind Singapore on the cards but just with the speed of the recovery it was definitely a possible thing.
"We sat long and hard to think about it, shall we do it or not, and I feel that I am ready. We will have to wait until tomorrow to see where it's at because driving around here is a bit of a different beast."
Russell said he had been concerned for Albon's welfare over the Italian Grand Prix weekend, after becoming aware of the problems his friend was having.
Albon had surgery for appendicitis on the Saturday of the Monza race and was sedated and put on a respirator after stopping breathing following the operation. He was taken off it in time to watch the race the following day.
"I was in contact with his family on Saturday night," Russell said, "because it was looking very scary at one point.
"But it's pretty impressive to see how he recovered so quickly. The human body is a scary thing. It just goes to show one minute everything is fine and the next minute everything can change almost totally out of your control. It will be interesting to see how he gets on this weekend."
Singapore is the most demanding event of the season for drivers because it is the longest race at nearly two hours, on a long, bumpy track, in a tropical environment with high humidity.
Albon said: "In terms of the surgery side, I am not worried about that at all. I know that's fully recovered. It's more the after effects of being in intensive care and the toll it has on your body. But I wouldn't be here if I didn't feel ready to race.
"We'll see how it goes, we're realistic, and know we're coming to the most difficult race of the year so we do have to be mindful of that. But I feel good. I have been karting and it's felt OK."
Russell described the difficulties for the drivers of racing in Singapore.
"It doesn't matter how much training you do, you will never be able to replicate what you go through on track," he said.
"I have been training with at least three layers of clothes on every single gym session, every time I go out on the bike.
"It's pretty uncomfortable. It's quite impressive how difficult the body handles heat, even in the sauna for half an hour, even though you're not moving, my heart rate was well over 150-160. That's what we will be experiencing in the car. And then there is the physical element and cognitive side of things."