The Finn, who won the season opening Australian Grand Prix, clocked a 1m36.569s to top practice ahead of Sebastian Vettel, Felipe Massa and Fernando Alonso.
A heavy shower approaching the hour mark ensured that the 1m36.569s that he had done on medium tyres 36 minutes in would stand as the fastest lap of the day.
Raikkonen's Lotus, Vettel's Red Bull and Massa's Ferrari had all taken turns in front during the dry section, and they ended up filling the final top three, covered by just 0.092 seconds.
The rain did not mean the end of the action. With similar conditions likely in both qualifying and the race, teams swiftly sent their drivers back out to sample both the wet track and the crossover to dry weather so they had some experience in case of rain later in the weekend.
The warm ambient temperature swiftly dried the track sufficiently for slicks, but the track was never quite good enough for drivers to beat their early times.
Behind the top three, Fernando Alonso edged out Mark Webber for fourth, with Romain Grosjean next up.
The Mercedes of Nico Rosberg and Lewis Hamilton were seventh and ninth, split by Paul di Resta.
Adrian Sutil held on to 10th despite a mechanical problem keeping his Force India in the garage during the rain delay.
Nearly everyone managed to avoid drama when the rain came, although Nico Hulkenberg did spin his slick-shod Sauber in the pitlane entry. Giedo van der Garde rotated on the track, spinning his Caterham onto the Turn 14 run-off.