Highlights - Belgian GP second practice
Belgian Grand Prix, 27-29 August, Spa
Coverage: Live on BBC One, Red Button, BBC Sport website and Radio 5 live
Full details of practice, qualifying and Sunday's race
Fernando Alonso posted the quickest lap times in each of Friday's two practice sessions for the Belgian Grand Prix.
The Ferrari driver signalled his intent with a lap of two minutes, 0.797 seconds in a rain-affected morning run.
He then capitalised on a chaotic end to a mostly dry afternoon stint to go round in 1:49.032, 0.125 seconds ahead of Force India's Adrian Sutil.
McLaren's Lewis Hamilton was third but Red Bull's championship leader Mark Webber was 2.6secs off the pace at Spa.
"It has definitely been a positive day because the car seems to respond well in all track conditions that we encountered," said Alonso.
"I think the weather will play a key part, both on Saturday and on Sunday. Anything could happen."
The Renault of Robert Kubica, testing the team's new F-duct aerodynamic system, came through fourth fastest and Alonso's team-mate Felipe Massa was sixth.
Red Bull's Sebastian Vettel, third in the drivers' standings, was 0.657secs off the pace and reigning world champion Jenson Button of McLaren was a further 0.066secs back.
Alonso came out on top but his rivals may point to the open track on which he was able to post his top time following a red flag towards the end of second practice.
Alonso happy with performance
Race control stopped the session because of spectators in a dangerous area - reports suggested children were attempting to climb on to a fence - and by the time cars were allowed back out, less than five minutes remained.
The drivers had little chance to warm their tyres up and embark upon a flying lap but Alonso found himself at the head of the pack and, while his rivals were denied a clear run, the Spaniard sped off.
"It was a bit of a messy day so I won't be losing sleep over it," said Webber, who is four points clear of Hamilton in the drivers' standing.
"It was actually a pretty good day for us, we worked through our programme and the car ran well. The weather conditions are changing and mixing it up a lot so it's very hard to get a feel on what anyone is doing.
"There are a few quick guys out there but we're not exactly slow either. It looks like an interesting weekend and we'll see [in qualifying] on Saturday, which is obviously a bit more important."
With teams not obliged to reveal the exact fuel loads the cars are carrying, it is difficult to read too much into the
- especially from a morning session blighted by incessant rainfall and an afternoon session that ended in bizarre fashion.
We had pretty much everything Spa could throw at us today and I'm happy with how the car feels in every condition - it was quite positive
The drivers were back in action for the first time since the Hungarian Grand Prix on 1 August following Formula 1's summer break, which included an enforced two-week factory shutdown when car development was prohibited.
So the day provided teams with a chance to regain familiarity with their vehicles, put any upgrades to the test and get to grips with the fast, challenging Spa-Francorchamps circuit.
Located in the mountainous Ardennes region of Belgium, this track is no stranger to adverse weather conditions and so it proved as the puddles of standing water and spray created a real problem early on.
But the sun eventually began to emerge and Alonso, who completed 17 laps to Hamilton's seven and Webber and Vettel's 11 in the morning, set his quickest lap.
"We waited for the track to dry and then out on the dry the car felt fairly good," stated 2008 champion Hamilton, whose McLaren MP4-25 should be suited to the first and third sectors at Spa due to its superb straight line speed.
Hamilton's car is suited the long straights in the first and third sectors
"It's way too early to say what's going to happen but I have positive feelings in the car and we're definitely making steps forward.
"We are always in the hunt but I definitely do feel that this track suits this car a little bit better and the car felt quite good today. If it rains then we should be competitive but we will have to wait and see."
The track temperature increased significantly for the start of second practice and Alonso led the way in switching from full wet tyres to intermediates, the move paying off as the two-time world champion posted the first sub-two minute lap of the weekend.
Most of the field followed suit and Hamilton, Massa, Kubica and Sutil all posted fastest laps before Torro Rosso's Jaime Alguersuari moved to the harder compound slick tyres and went quickest by almost three seconds.
Button and Hamilton were among those to respond by pitting for the softer compound slicks and leaping atop of the timesheets, only for their progress to be halted by the red flag.
"It wasn't too bad at least we got a bit of wet running, intermediate running and a bit of dry running," commented Button. "We had pretty much everything Spa could throw at us today and I'm happy with how the car feels in every condition. It was quite positive.
"Qualifying [in these conditions] will be complicated and very messy. We have to make sure that if it is tricky conditions in Q1 we are out on the circuit at the correct time - but we won't make the same mistake we made in
Williams driver Rubens Barrichello, who is set to become the first Formula 1 driver to contest 300 races on Sunday, was almost 1.1secs off the pace.
Mercedes' seven-time world champion Michael Schumacher goes into the race with a 10-place grid penalty after nearly running Barrichello into a pit wall at the Hungarian Grand Prix.