Pastor Maldonado set the pace for Williams during Friday's opening practice for Sunday's European Grand Prix here.
The surprise Spanish Grand Prix winner clocked his fastest time midway through the session after drivers had seemed reluctant to go out on track because of the overcast conditions and unexpected track temperature.
The 27-year-old covered the Valencia street circuit in one minute 40.890 seconds ten minutes before the hour mark on fresh rubber and warming track conditions.
Maldonado's time was roughly 0.4 seconds slower than last year's pace-setter, Australian Mark Webber of Red Bull, who was at that time running the controversial rear-blown diffuser on his car.
Defending drivers' champion Sebastian Vettel was second fastest for Red Bull ahead of his team-mate Webber and fourth-fastest Briton Jenson Button of McLaren.
Ferrari driver Fernando Alonso of Spain was fifth fastest ahead of Briton Paul di Resta of Force India.
Canadian race winner Lewis Hamilton of Britain was eighth fastest in the second McLaren ahead of Mercedes driver Nico Rosberg of Germany and Finland's Kimi Raikkonen of Lotus, who completed the top ten.