Formula One 'Iceman' Kimi Raikkonen lapped fastest for Lotus in the Bahrain desert heat on Friday after Ferrari had set the early practice pace.
The Finn, 2007 World champion for Ferrari and chasing his 21st successive race in the points, sped to a best time of one minute 34.154 seconds on the medium tyres in the afternoon session when track temperatures hovered around 42 degrees celsius.
"It's nice to be fastest but you never know what the others are doing; we just stick to our programme and don't take too much notice of what else is happening," said Raikkonen, winner of the season-opener in Australia last month.
"I actually made a mistake through the final corners on my fastest lap, so there's still more time to be found."
Practice for the most controversial race of the season went ahead untroubled by thousands of anti-government protesters who took to the streets of the Gulf kingdom to press for political change.
The Sakhir circuit is some 30km southwest of the capital Manama and ringed by tight security.
Brazilian Felipe Massa kept Ferrari team mate Fernando Alonso off the top in the morning with a lap of 1:34.487.
The Spaniard, winner of the third race of the season in China last weekend, was 0.077 slower. The Ferrari pair focused on long runs in the afternoon, one on the medium tyres and the other on hards.
Massa then switched to the hards after complaining about wear on the rears.
"We had no major problems and more importantly, we didn't have to make any radical changes to the F138 to adapt it to this track and its long straights and slow corners, characteristics which I like a lot," said Alonso.
Technical director Pat Fry said some of the aerodynamic updates tested had worked well but others proved inconclusive and might not be used.
Germany's Nico Rosberg was third fastest in the morning for Mercedes, who had Lewis Hamilton only 13th, with Red Bull's triple champion and overall leader Sebastian Vettel taking third after lunch.
Rosberg expected the race to be very hard on the rear tyres and said the team had a lot of work to find a good balance for Sunday's race.
Hamilton, 10th in the second session, predicted a tough weekend ahead.
Australian Mark Webber, making his 200th start this weekend with a three-place grid penalty after a nightmare race in Shanghai, was seventh and second for Red Bull in the two sessions.
"If you look at the opposition then Ferrari and Lotus were strong today and I think it will be close in qualifying," said Vettel, who was unhappy with his short runs.
McLaren's Jenson Button was sixth and 11th while Mexican team mate Sergio Perez was 11th and 13th.
"I've already got a good feeling that we've found a set-up and a direction for tomorrow that will be more competitive," said Button. "There's a lot we can take away from today that will help us for tomorrow."
Finland's Heikki Kovalainen, dropped by Caterham at the end of last season, made his return to the team as a reserve driver and lapped 20th of the 22 drivers in the first practice.
"The first impression I had was that this car...is certainly trickier to find a balance on than the 2012 car, and the 2013 tyres go off a lot faster than last year's," he said.
Another reserve, Venezuelan Rodolfo Gonzalez, made his first Friday appearance for Marussia in place of Jules Bianchi but completed only seven laps due to a gearbox problem.
Team principal John Booth said he had done a good job and more opportunities would come his way.
Photograph: Clive Mason/Getty Images
