Lewis Hamilton continued the Mercedes team's dominance of the 2014 Formula 1 season by topping first practice for the Bahrain Grand Prix.
The Malaysian GP winner was fastest during the opening half-hour of the 90-minute session, during which drivers are permitted to use their extra set of medium-compound Pirellis, lapping 0.113s faster than team-mate Nico Rosberg.
Both Mercedes waited until the final 20 minutes of the session to head out on their second set of mediums, by which time Ferrari's Fernando Alonso had taken top spot after setting a time four tenths of a second faster than Hamilton's first run despite a brief lock-up at Turn 10.
Hamilton then set the fastest time of the session in all three sectors to reclaim top spot, the second time he has been quickest during FP1 in three 2014 races.
Rosberg retained second place, although he was around a tenth slower than Hamilton had been on the first runs, ending up 0.231s off the pace.
Alonso ended the session third fastest, four-and-a-half tenths slower than Hamilton and 0.169s ahead of Nico Hulkenberg as the Ferrari and Force India drivers continued the battle that has raged throughout the season so far.
The German looked to have the potential to outpace the Ferrari after lapping a quarter-of-a-second faster than Alonso in sector one but lost time over the rest of the lap.
Alonso endured an abortive start to his running on his second set of rubber, Ferrari inadvertently sending him out of the garage with one soft tyre and three mediums. He stopped in the pitlane and was dragged back to the garage to replace the soft.
Jenson Button's McLaren was fifth fastest, just ahead of the second Ferrari of Kimi Raikkonen, who had a big lock-up at Turn 1 and ran off the track while on his long-run on his second set of mediums.
The second McLaren of Kevin Magnussen was seventh fastest, ahead of Daniil Kvyat's Scuderia Toro Rosso, the 19-year-old Russian ended up 1.5s off the fastest time.
World champion Sebastian Vettel was only 10th fastest behind the second Force India of Sergio Perez, 1.887s slower than Hamilton, with team-mate Daniel Ricciardo a second slower in 14th.
Felipe Massa was the quickest Williams, setting the 11th fastest time after completing only one installation lap on his first set of tyres.
Three reserve drivers made their first appearances of the season during the session, with Felipe Nasr, driving Valtteri Bottas's Williams, the quickest of them in 13th place.
Sauber's Giedo van der Garde, who took over Esteban Gutierrez's Sauber, was 18th fastest, with Caterham's Robin Frijns 21st quickest in Kamui Kobayashi's car.
Pos Driver Team Time Gap Laps 1. Lewis Hamilton Mercedes 1m37.502s 14 2. Nico Rosberg Mercedes 1m37.733s +0.231s 13 3. Fernando Alonso Ferrari 1m37.953s +0.451s 17 4. Nico Hulkenberg Force India-Mercedes 1m38.122s +0.620s 10 5. Jenson Button McLaren-Mercedes 1m38.636s +1.134s 16 6. Kimi Raikkonen Ferrari 1m38.783s +1.281s 12 7. Kevin Magnussen McLaren-Mercedes 1m38.949s +1.447s 15 8. Daniil Kvyat Toro Rosso-Renault 1m39.056s +1.554s 24 9. Sergio Perez Force India-Mercedes 1m39.102s +1.600s 21 10. Sebastian Vettel Red Bull-Renault 1m39.389s +1.887s 16 11. Felipe Massa Williams-Mercedes 1m39.533s +2.031s 11 12. Jean-Eric Vergne Toro Rosso-Renault 1m39.862s +2.360s 26 13. Felipe Nasr Williams-Mercedes 1m40.078s +2.576s 14 14. Daniel Ricciardo Red Bull-Renault 1m40.406s +2.904s 19 15. Adrian Sutil Sauber-Ferrari 1m40.652s +3.150s 20 16. Pastor Maldonado Lotus-Renault 1m40.793s +3.291s 31 17. Jules Bianchi Marussia-Ferrari 1m40.$89s +3.387s 20 18. Giedo van der Garde Sauber-Ferrari 1m40.913s +3.411s 20 19. Romain Grosjean Lotus-Renault 1m41.036s +3.534s 24 20. Max Chilton Marussia-Ferrari 1m41.794s +4.292s 20 21. Robin Frijns Caterham-Renault 1m42.417s +4.915s 35 22. Marcus Ericsson Caterham-Renault 1m42.711s +5.209s 21