Lewis Hamilton (GBR) Mercedes AMG F1 takes the ice bucket challenge following his nomination by Daniel Ricciardo (AUS) Red Bull Racing. Formula One World Championship, Rd12, Belgian Grand Prix, Practice, Spa-Francorchamps, Belgium, Friday 22 August 2014. © Sutton Motorsport Images
Rosberg, the championship leader, went fastest early on with 1m 51.577s, a time that Hamilton failed to beat by 0.097s with 1m 51.674s before the pair shifted their focus to race preparations.
Both had early minor moments at the Bus Stop chicane, while the Briton also had to save a big sideways moment as he negotiated the fearsome Eau Rouge. F1 racing's most famous corner is much more of a challenge this year, with the reduction in downforce.
As usual on a Friday, Alonso got his F14 T close to the silver cars with 1m 51.805s, while Button got going after early delays to take his McLaren round in 1m 52.404s for fourth.
There was a gap then to Kimi Raikkonen on 1m 52.818s in the second Ferrari, with Sergio Perez and Nico Hulkenberg sandwiching Kevin Magnussen in the Finn's wake. The Mexican lapped his Force India in 1m 52.903s, the German his in 1m 52.937s, while the Dane in the middle turned a lap of 1m 52.922s in the second MP4-29. Daniel Ricciardo concluded the sub-1m 53s runners, with 1m 52.972s in the lead Red Bull.
Williams, who have high expectations for this weekend, had a relatively slow start, with Valtteri Bottas unable to better 1m 53.172s for 10th and Felipe Massa down in 15th on 1m 53.968s.
It wasn't a great morning for Sebastian Vettel either. The world champion had problems with the upshift from second to third and third to fourth, and then ended his session in the garage after completing just 11 laps - the best of which was a 1m 53.369s that left him 11th.
Daniil Kvyat was easily Toro Rosso's leader in 12th place on 1m 53.594s but complained of brake problems late in the session, as Jean-Eric Vergne was only 16th on 1m 54.189s.
Between them, Romain Grosjean was 13th for Lotus on 1m 53.597s, ahead of Adrian Sutil's Sauber on 1m 53.703s and Massa.
Giedo van der Garde took over the cockpit of Esteban Gutierrez's Sauber for the session but struggled with hesitant power delivery on his way to 17th fastest time of 1m 54.335s which left him ahead of Pastor Maldonado in the second Lotus on 1m 55.336s.
Jules Bianchi got close to the Venezuelan with 1m 55.782s in the faster Marussia, while the second of the Banbury team's cars was the subject of much speculation all morning. Alex Rossi drove it as intended in this session, lapping in 1m 57.232s for 20th slot, but even before he got going he learned that he will not make his Grand Prix debut this weekend after all as Max Chilton's contractual problems had been resolved. The Englishman will thus be back in the car for the rest of the meeting.
At the back, the revised Caterhams were half a second off Rossi and a long way off Bianchi. On his F1 debut Andre Lotterer did a decent job to lead team mate Marcus Ericsson, with respective times of 1m 57.886s and 1m 57.997s.
There were several minor incidents in the 90-minute session. Raikkonen and Ericsson both spun at La Source, whilst Lotterer, Button, Alonso, Sutil, Kvyat and Van der Garde all had off-track excursions.