Renault's Fernando Alonso, whose participation in Formula One's Valencia grand prix race this weekend was confirmed only days ago, topped the timesheets in practice.
The double world champion lapped the Mediterranean port's street circuit with a quickest time of one minute 39.404 seconds.
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.
Ferrari stand-in Luca Badoer made a slow start to his Formula One comeback and was then fined and reprimanded for being too fast in the pitlane.
The 38-year-old Italian, who last raced 10 years ago, collected three separate fines totalling over 11 thousand dollars before finally being hauled before the race stewards for a fourth offence.
The infringements were all within a half hour period in the day's second session.
The test driver was elevated to the race seat after Brazilian Felipe Massa was seriously injured in Hungary last month and retired champion Michael Schumacher was ruled out for medical reasons.
Badoer finished up 18th and ahead of McLaren's world champion Lewis Hamilton.
