Lewis Hamilton set a stunning pace in second practice at the Belgian Grand Prix to lead team-mate Nico Rosberg to a Mercedes one-two.
Hamilton was 0.604 seconds quicker than the German, who is 11 points ahead in the championship heading into this race, the first after the summer break.
Ferrari's Fernando Alonso was a shock third ahead of Williams' Felipe Massa and McLaren's Jenson Button.
But the Spaniard was 0.741 seconds adrift of Hamilton's lap.
Rosberg made a small mistake exiting the Malmedy right-hander at the top of the hill on his lap, running slightly wide on to the kerb, but BBC F1 analyst Allan McNish reckoned that had cost him only 0.1secs.
Hamilton entered the weekend saying he felt "amazing" and "really energetic" after the break, and determined to start the second and decisive part of the season strongly after a series of problems blighted his early campaign.
And his stunning lap was a powerful statement of intent, especially as he had appeared to struggle slightly in the first session.
He was second to Rosberg, by only 0.097secs, but had a series of incidents, missing a couple of braking points and running wide twice at the 150mph Pouhon double-left.
Hamilton also had to rescue a slide at the 180mph+ Eau Rouge sweeps.
He put that behind him in the afternoon, though, and was fastest on the harder 'medium' tyre early in the session by about 0.4secs, before stretching that margin even further on the 'soft' tyre that will be used for qualifying.
Hamilton also appeared to have a slight edge on race pace, judging by the long-run times on heavy fuel, set later in the session.
Mercedes' closest rivals were expected to be Williams, and may remain so, but Ferrari appeared more competitive on Friday than they had thought they would be.
Alonso entered the race weekend saying he expected this event and the subsequent one in Italy to be "damage limitation".
|2014 Formula 1 season so far|
|Lewis Hamilton||Nico Rosberg|
The team have an update on their engine, adding insulating material to the exhaust pipes in the hope of retaining more heat and thereby increasing the effectiveness of their engine recovery system.
But Ferrari have often flattered to deceive on Fridays this season, only to drop back in qualifying.
"I think Ferrari will be strong this weekend," said McNish. "The circuit suits the characteristics of their car. I think [Alonso] could be a surprise tomorrow afternoon, perhaps even grab a place on the second row."
"Mercedes will be strong as always, but it will be Ferrari right behind and then a good battle between Williams and Red Bull."
Behind Button, Williams's Valtteri Bottas was sixth, ahead of Toro Rosso's Daniil Kvyat, Ricciardo, McLaren's Kevin Magnussen and Force India's Nico Hulkenberg.
Ricciardo's team-mate, world champion Sebastian Vettel, was forced to miss the session because a failure in the car's hybrid system meant Red Bull had to change his engine.
Vettel admitted it was now inevitable he would sooner or later have to suffer at least one mandatory 10-place grid penalty for using more than his allowed five engines this season. "The only question is how many," Vettel said.
An eventful session was stopped twice, first for a crash by Lotus's Pastor Maldonado on the hill down towards the fast left-hander at Pouhon and then for a spin at Blanchimont by Sauber's Esteban Gutierrez.
Maldonado lost control after getting the car halfway on to the grass after leaving the Liege left-hander and failing to bring it back on to the track.
The Venezuelan was taken to a hospital for a check-up but was none the worse for his misadventure.
Gutierrez blamed a gearbox problem for his spin at the 180mph Blanchimont bend, saying: "It's the gearbox. I turned in and something completely braked the car."