Germany's Nico Rosberg who has yet to win a grand prix and celebrates his 25th birthday on Sunday, lapped the Valencia street circuit with a timesheet-topping best of one minute 41.175 seconds.
Germany's Nico Rosberg denied Lewis Hamilton and Jenson Button another one-two in first free practice for the European Grand Prix today.
The Mercedes driver, who has yet to win a grand prix and celebrates his 25th birthday on Sunday, lapped the Valencia street circuit with a timesheet-topping best of one minute 41.175 seconds.
Championship leader Hamilton, winner of the last two races in successive one-twos with McLaren team mate and reigning world champion Button, was second quickest 0.164 behind Rosberg.
Button was third and 0.208 slower.
Mercedes are one of several teams with major developments to the car this weekend, bringing an ''F-duct'' similar to that used to telling effect by championship leaders McLaren since the start of the season.
The duct is operated by the driver to stall the rear wing, allowing a higher top speed on the straights.
Rosberg's team mate, seven times world champion Michael Schumacher, was only eighth fastest however with Red Bull's German Sebastian Vettel and Australian Mark Webber sixth and seventh.
Renault and Ferrari, who also have significant aerodynamic upgrades, looked competitive with Poland's Robert Kubica fourth for the former and Brazilian Felipe Massa -- winner in Valencia in 2008 -- fifth for the latter team.
The session was halted when a mirror fell off Bruno Senna's HRT and was run over by Toro Rosso's Sebastien Buemi, leaving debris on the track.
Indian Karun Chandhok, Senna's team mate, sat out the session with Austrian test driver Christian Klien given some mileage instead.
Britain's Paul di Resta, Force India's reserve, also made a return to Friday practice in the place of regular race driver Adrian Sutil.