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Sebastian Vettel has admitted he was not sure if he was up to the job of being a Formula One driver when he first tested a Williams in 2005.

Vettel's debut in an F1 car came at a test in Jerez in September 2005, when he shared some seat time with Mark Webber and Nico Rosberg in a Williams-BMW FW27. Just 18 years old at the time, he only completed 25 laps and was over three seconds off Webber's benchmark time. Now close to his fourth world title, he admitted the step up from Formula 3 was daunting at the time.

"The first time I tested the car, Mark drove in the morning and I drove in the afternoon," he said. "I shat myself for the first couple of laps and I thought, alright, that's for real men, not for me. Then I got used to it and obviously wanted to do more."

Since then Vettel has racked up 35 wins and is now closing on his fourth consecutive world championship. Nevertheless, he still gets nervous on race day mornings.

"Nothing has changed in the way that I still love racing, I love the challenge, I'm still nervous when I wake up on Sunday, still excited when I walk on the grid and tense, looking forward to the race," he added.

"I think it would be a shame if you were too tense and if you tried to force things too much. I think you have to allow yourself to enjoy it because this is not normal, it doesn't happen to everyone. I think I'm very fortunate that I'm one of these 22/24 guys in Formula One."

Vettel needs to finish fifth or higher at next week's Indian Grand Prix to secure the title.

© ESPN Sports Media Ltd.