The Finn had been scheduled to take over from Romain Grosjean for Saturday and Sunday.
But he was struck with suspected food poisoning and had to take Saturday to recover.
Grosjean had already left the track, so reserve driver Davide Valsecchi was called up to begin running for Lotus until the Frenchman made it back to Barcelona.
Lotus team boss Eric Boullier said on Saturday night that Raikkonen was expected to be fit enough to drive on Sunday, and he duly took to the track when the session began.
Grosjean remains on standby to take over in case of further problems.