Britain's Anthony Davidson will drive for BAR-Honda in Sunday's Malaysian Grand Prix after team-mate Takuma Sato withdrew on medical advice.
Davidson has been given a chance to shine in Malaysia
The Japanese developed a fever on Friday and the team were advised not to allow him to race because of the high temperatures and humidity at Sepang.
Davidson was 15th fastest in first qualifying in his first race since he did two races for Minardi in 2002.
"This is probably the toughest challenge I've ever faced," he said.
"Although I know the car very well, I'm still learning the track because I had limited running on the Friday here last year.
"It's always tough being the first car out (in qualifying) and I wanted to take it a bit steady and keep the car on track.
"Overall I'm pleased with my performance under the circumstances and I'm confident that there's more to come in Sunday's final qualifying run."
"It's been a tough but exciting day for me. When I had the final confirmation that I would be driving, I was packing my bags ready to go home!
Davidson, 25, has been BAR's test and reserve driver since 2001.
"Obviously it's jumping straight into the thick of it," said BAR technical director Geoff Willis on Saturday.
"Neither he, nor we, were expecting him to be in the car this morning.
"Anthony arrived on Thursday evening but he is very, very fit at the moment. He's really quite robust and we're sure he'll be up to it."
Sato said: "I am obviously extremely disappointed that I have had to retire from the Malaysian Grand Prix - a race that I have prepared extremely hard for.
"Having discussed my condition with the medical staff and the team, I accept their advice that it is not sensible for me to take part in such a physically demanding race.
"I have now been advised to get plenty of rest and look forward to getting well again in time to be back in the car for the next race in Bahrain."