Vitaly Petrov says he is still uncomfortable inside Caterham's F1 car

The 27-year-old Russian, who first drove for the team at Barcelona last week, was unexpectedly called up to drive on the opening day of the final pre-season test at the same circuit on Thursday after his team-mate Heikki Kovalainen came down with food poisoning.

Petrov completed 123 laps during the opening day, which came to an early end after he stopped out on track during a dry fuel run and brought out the red flags with half an hour remaining.

Despite ending up ninth quickest - ahead of Pastor Maldonado's Williams and less than 1.7s off Romain Grosjean's pace-setting time - he insisted that he will not be able to extract the maximum from himself or the car until his new seat is delivered.

When asked by AUTOSPORT about comfort levels in the car, Petrov said: "At the last test it was my seat, but it was not fitting properly. The guys made something with the seat that was quite uncomfortable and I had a lot of pain in the back and shoulders.

"Here it is not perfect, but it's better. My new seat gets delivered on March 3 [Saturday], so I can use it on Sunday when I drive.

"It affects your concentration because you start to brake and you get something pinching you, and then you're in the corner. Of course you lose concentration. Things will be much better on Sunday."

Petrov, who was originally scheduled to drive on the final two days of the test, was only informed two hours before testing got underway that he would be replacing Kovalainen.

"I got a call at 6:40am. It's strange because usually I turn off my ring tone at night, but I forgot, so the call came through," he added. "The first time I didn't take it, but when I saw who it was I realised it could be important.

"It wasn't a problem. I always get to the track for the 8am briefing, so I would have been up by 7am. He'll be back in the car on the second day though."