Williams Formula 1 driver George Russell says he is fit enough to complete the Abu Dhabi Grand Prix after concerns over his fitness led to Sergey Sirotkin being on standby.
Russell reported to Williams on Thursday that he was not feeling fit and the team searched the paddock for a replacement.
Mercedes reserve Esteban Ocon had a seat fitting, but his long legs meant he was not able to fit comfortably in the car with Russell's custom-made carbon seat.
There was no opportunity to make a seat for Ocon, meaning Williams turned to its 2018 race driver and Renault/McLaren reserve Sergey Sirotkin.
Russell was then passed fit for FP1 by Hintsa Performance staff, who look after his medical and fitness requirements, and the driver completed the two sessions without issue.
Asked by Autosport to explain the issue that had caused doubts to his participation, Russell said: "I won't expand on it.
"It's just something different. I'll expand when the time is right, but not now.
"How you feel when you're sat at home on your hotel bed, and how you feel when you are driving are two very different things.
"I felt alright last night, I didn't know how I'd feel today.
"It will all make sense when I explain, but as I said, the time is not right now.
"Number one priority is looking after myself, and it was always the idea that we'd assess after practice.
"Obviously the team need a back-up plan, and they have that.
"I don't feel 100%, but I'm well enough to drive.
"I've not been feeling great, the guys at Hintsa are overlooking everything.
"I'm feeling good to drive, so that's the most important thing."
Team boss Claire Williams acknowledged the team had a back-up plan, but declined to confirm whether it involved Sirotkin.
"I think it's the end of the season, isn't it?" she told Sky F1. "I think everybody's feeling a little jaded.
"I think whenever you have a driver you're a little concerned about, of course, you've got to have some level of contingency, so we did that, we made sure that we did."
Russell and team-mate Robert Kubica finished the second practice session at the bottom of the timesheets.
"The pace was quite slow, so we need to understand why that is," said Russell. "But the car was feeling nice to drive.
"It seems when the car is feeling good we're slow, and when the car is not feeling good, we're relatively fast.
"I was quite pleased with FP2, but it's all about the lap time, and the lap time wasn't there."