McLaren's Lando Norris revealed after finishing eighth in the Spanish Grand Prix that he had been fighting tonsillitis all week and racing had been a huge struggle.
The 22-year-old saw a doctor immediately afterwards for a full health check and was unable to attend his usual media duties.
Television images immediately before the start, with ambient temperatures in the high-30s, had also showed the Briton looking unwell.
"Today was tough. I was feeling really unwell before the race as I'm suffering with tonsillitis, and that, in combination with the high temperatures, made this one of the hardest races I've ever done," he said in a team statement.
"I've been a bit on the backfoot this weekend as most of my energy has been spent fighting off this illness.
"I've had to miss a lot of engineering sessions, which has compromised my weekend, and I definitely wasn't as prepared for the Grand Prix as I could have been," he added, after starting in 11th place.
With that in mind, I'm really pleased to have come out of the race with decent points for the team."
Norris is seventh overall in the championship, with McLaren fourth in the constructors' standings and Formula One racing in Monaco next weekend.