Sergio Perez felt dizzy on Friday after returning to the wheel of his Sauber.
The Mexican rookie pulled out of the recent Canadian Grand Prix weekend when he felt dizzy, two weeks after his heavy qualifying crash in Monaco.
But his Swiss employer said on Friday that Perez, 21, will contest the rest of the event on the streets of Valencia.
"At the beginning of the first session I still felt dizzy," Perez told the Spanish news agency EFE, "but as it went on I felt better and in the end I felt good.
"Tomorrow (Saturday) I hope to be 100 per cent. I think it's a normal situation as the body gets slowly used to it again. The second session was much better," he explained.
Perez said he had trained and prepared hard for his Valencia comeback, almost a month after his hospitalisation in Monaco.
"I think we have succeeded," he added.