Ferrari's Felipe Massa and Fernando Alonso topped the opening practice session for the Bahrain Grand Prix on Friday as they finished one-two split by just 0.077s.
Nico Rosberg was third fastest, 0.134s off the pace, while Sebastian Vettel was fourth fastest as Ferrari, Mercedes and Red Bull look set to do battle again this weekend. Paul di Resta showed some promising pace in the Force India with the fifth fastest time ahead of Jenson Button, who dragged his McLaren to within 0.6s of Massa's fastest time.
Mark Webber was seventh fastest ahead of the second Force India of Adrian Sutil, which ran two different types of floor to gather data on the VJM06's latest update. The Lotuses of Kimi Raikkonen and Romain Grosjean rounded out the top ten, with Raikkonen 0.858s off the pace and Grosjean 1.124s off. Lotus replaced the chassis on Grosjean's car after persistent balance problems at the first three races, but also had to address a KERS issue, limiting the Frenchman's mileage slightly compared to his team-mate.
Sergio Perez was 11th fastest in the second McLaren as he again struggled to match the pace of team-mate Button. However, he appeared to have a problem in the final 30 minutes of the session as McLaren mechanics swarmed around the rear of his car addressing an issue.
Valtteri Bottas was 12th fastest in the Williams and ahead of Lewis Hamilton in the Mercedes, who did not appear to attempt a fast time. On one of his laps Hamilton was faster than Massa through the first sector, but then backed off and returned to the pits.
Jean-Eric Vergne was 14th ahead of Toro Rosso team-mate Daniel Ricciardo, while Pastor Maldonado was 16th and the two Saubers of Nico Hulkenberg and Esteban Gutierrez 17th and 18th. Charles Pic finished the session 0.5s faster than Heikki Kovalainen, who returned to the cockpit of the Caterham for the morning session to help the team analyse its new development parts. The Caterhams were over a second faster than rivals Marussia, led by Max Chilton while reserve driver Rodolfo Gonzalez completed just seven laps on his Friday morning practice debut with the team.