Sauber's Sergio Perez concedes he will have to wait until after tomorrow's first practice session for the European Grand Prix before knowing if he is fit to race this weekend.
Perez is convinced he is now fully able to compete in Valencia after missing the last two races following his accident in qualifying in Monaco nearly four weeks ago.
The 21-year-old Mexican sustained concussion after hurtling into a barrier at the Harbour chicane that resulted in him spending two nights in hospital.
Ahead of the Canadian Grand Prix, Perez was declared fit by FIA medical delegate Jean Charles Piette and medical officers at the Circuit Gilles Villeneuve.
But after complaining of feeling unwell after first practice, Perez pulled out of the race, forcing Sauber into drafting in McLaren reserve Pedro de la Rosa, who competed in 13 grands prix for the team last year.
"The doctors always thought we were a bit too optimistic to go back after the accident because it was a very big one," said Perez.
"I had never had one like that before, and I think you need quite a bit of time to recover because it was a bang to the head.
"After the first session, although my time was quite competitive, I was not feeling 100%, not feeling good at all.
"With the change of direction I was feeling very dizzy, so together with the team we decided to wait a bit more and return when I am 100% right.
"I have done a lot of training to get back, and I'm motivated more than ever to do so. I'm looking forward to being back this weekend."
Despite the additional two weeks of recuperation, rookie Perez knows it is tomorrow's first practice session that will tell him all he needs to know.
"It is an important day, especially the first practice to make sure everything is fine," added Perez, who watched the last race at home in Mexico.
"It is all up to me, as it was in Montreal. The doctors told me if I was not feeling right to come and speak to them.
"There won't be any more checks, they all trust me, and if I feel fine I will drive. If I still feel dizzy, I won't.
"If I feel 100% inside the car, I think I will be as quick as I was straight away in Canada.
"I am quite positive everything will be okay."