Williams' Felipe Massa locks up his brakes at Turn 3 and runs into the gravel trap, but is able to return and continue. © FOWC Ltd
In cool, overcast but dry conditions, the fastest time initially see-sawed between the Mercedes drivers as Rosberg lapped in 1m 15.162s to Hamilton's 1m 15.700s, then Hamilton improved to 1m 13.261s to Rosberg's 1m 13.508s, and then improved further to 1m 12.255s.
On the second runs, however, Rosberg managed 1m 11.295s to take fastest time away from Jenson Button, who'd lapped his upgraded McLaren in 1m 11.839s. It was at this point, 40 minutes into the session, that it began to drizzle for the first time, although barely ten minutes later conditions had improved enough for drivers to start going faster again.
Hamilton moved up to second on 1m 11.435s and Alonso third in 1m 11.606s, only for the rain to briefly return once more. As conditions improved again towards the end everyone started pushing, but in general the frontrunners' times were still a second or so off.
Thus Rosberg and Hamilton remained top from Alonso and Williams' Felipe Massa on 1m 11.756s, with Button's time holding up for fifth.
Sergio Perez narrowly beat Force India team mate Nico Hulkenberg, with 1m 12.009s to 1m 12.072s for sixth and seventh, with Valtteri Bottas eighth for Williams on 1m 12.114s ahead of McLaren's Kevin Magnusson on 1m 12.313s, Toro Rosso's Jean-Eric Vergne on 1m 12.364s and Kimi Raikkonen in the second Ferrari on 1m 12.365s.
Daniil Kvyat was 12th in the other Toro Rosso, on 1m 12.372s, which meant that Canada winner Daniel Ricciardo was Red Bull's lead runner down in 13th on 1m 12.570s. Team mate Sebastian Vettel was only 15th on 1m 12.988s, his final effort just failing to dislodge Esteban Gutierrez who lapped his Sauber in 1m 12.984s.
The Lotuses were only 16th and 17th, Romain Grosjean lapping in 1m 13.168s to team mate Pastor Maldonado's 1m 13.642s. The Marussias were again closely-matched, with Jules Bianchi pipping Max Chilton 1m 13.738s to 1m 13.857s.
Kamui Kobayashi couldn't better 1m 14.611s for Caterham, which left him ahead of Adrian Sutil who had a troubled session for Sauber which ended with 1m 14.691s.
Marcus Ericsson stopped early on exiting Turn 1 as his Caterham switched itself off. It was recovered to the paddock, where it spent the rest of the session with its undertray off in the garage as the team attempted to discover what the CT05's telemetry refused to divulge. The Swede was 22nd, on 1m 17.501s.
The prize for the biggest moment of the morning was won hands-down by Vettel. Massa, Alonso, Magnussen, Grosjean, Bianchi, Bottas, Rosberg, Hamilton, Maldonado and Kvyat all had off-course moments after leaving their braking too late (mainly in Turn 1, partly because of a 10 km/h tailwind) or getting sideways, but Vettel got on to the grass exiting Turn 9 and performed two perfect 360 degree spins - without hitting the barriers - and finished with his RB10 perfectly lined up on the grey stuff to continue his lap.
Ricciardo, soon after, just got away with his own grassy adventure as Red Bull ended the first session at their home track some way off down the order.