Kimi Raikkonen sent out an ominous warning to his championship rivals in Barcelona after topping the time-sheets in both practice sessions, with Renault an unexpected presence in the afternoon.
The Finn was a full 1.3 seconds slower than he went in the morning session but retained an advantage over his team-mate Felipe Massa and the McLarens of Lewis Hamilton and Heikki Kovalainen.
Hamilton, who lapped third quickest in the morning session, half a second slower than Raikkonen, struggled with oversteer throughout the day and ended the afternoon languishing in eleventh place after spinning at Turn 10 on his hot-lap.
Things weren t much better for Kovalainen in the sister McLaren who completed just eight laps as a result of a gearbox problem.
But it was Renault that caused the biggest upset of the day with Nelson Piquet and Fernando going second and third quickest respectively in the second session, only a tenth of a second shy of Raikkonen.
The lap times yo-yoed around with no fewer than eight drivers occupying the top spot including, in the early stages, the Force India and Williams driver pairings.
Then with 27 minutes remaining Alonso lit up the timing-screens much to the delight of his adoring Spanish fans. The celebrations were temporary however with team-mate Nelson Piquet knocking the double world champion off the top spot by a scant one hundredth of a second shortly after.
Nakajima held on to fourth place for Williams, with Massa fifth and 0.2 seconds slower than Raikkonen.Mark Webber and David Coulthard took sixth and eighth for Red Bull, split by Rosberg, while the two Force Indias were demoted to ninth and tenth.
BMW Sauber kept a low key with Robert Kubica and Nick Heidfeld lining up twelfth and thirteenth followed by Trulli, Button, Kovalainen and Barrichello.
Toro Rosso and Super Aguri brought up the rear split by the Toyota of Timo Glock.