Renault's Fernando Alonso, whose participation in Valencia was confirmed only this week, topped the timesheets in practice for his home European Grand Prix today.
The double world champion lapped the Mediterranean port's street circuit with a quickest time of one minute 39.404 seconds, despite an earlier collision with BMW-Sauber's Nick Heidfeld.
Brawn GP's Formula One championship leader Jenson Button was second fastest, 0.774 slower than the Spaniard, with Brazilian team mate Rubens Barrichello third.
Barrichello had set the pace in the morning with a time of 1:42.460.
Alonso had been in danger of missing Sunday's race after his Renault team were handed a one-race ban in Hungary last month but the team had that overturned at an appeal hearing in Paris last Monday.
Ferrari stand-in Luca Badoer completed 62 laps, more than a race distance, in the two sessions.
The 38-year-old Italian, preparing for his first start in a decade after Brazilian Felipe Massa was seriously injured in Hungary, was more than a second slower than everyone else in the opening session.
However, with retired seven-times champion Michael Schumacher watching his friend from the pit lane wall, he lifted himself off the bottom in the afternoon to end up 18th and ahead of McLaren's world champion Lewis Hamilton.
The Briton had been third in the morning.
Schumacher, who retired in 2006 and is now 40 years old, would have taken Massa's place but the German had to abort his planned comeback due to a neck injury suffered in a bike crash earlier in the year.
Button's time sent a clear indication that his Mercedes-powered team have got to the bottom of the problems that kept him off the podium for the past three races.
The Briton, who leads Red Bull's Australian Mark Webber by a comfortable 18.5 points after winning six of the first seven races of the year, was quickest for much of the opening session.
France's Romain Grosjean made an assured debut after replacing Brazilian Nelson Piquet at Renault, lapping 17th and 13th fastest respectively.
Spanish rookie Jaime Alguersuari also put on a good show for the locals, although the grandstands looked very empty, with the 13th best time for Toro Rosso in the morning. He slipped to 19th after lunch.