Red Bull's Mark Webber set the pace from Ferrari's Fernando Alonso in a rain-interrupted practice session at the Belgian Grand Prix.
The Australian set a best lap of one minute 50.321 seconds to beat Alonso by just 0.140secs, with McLaren's Jenson Button and Lewis Hamilton next.
Button was 0.449secs adrift of Webber, with Hamilton 0.068secs further back.
Force India's Paul di Resta was ninth on his F1 debut at Spa, just behind the team's reserve driver Nico Hulkenberg.
Both sessions on Friday were disrupted by rain.
The second session started on a damp track after showers in the lunch interval, but it dried up sufficiently for a period of running on slick tyres in the middle of the session.
The drivers first ran on harder 'medium' tyres, on which World Championship leader Sebastian Vettel was fastest by 0.3secs from Hamilton.
Vettel did not go out on the soft tyres towards the end of the dry window, unlike Webber, Alonso, Button, Hamilton, Ferrari's Felipe Massa and Mercedes' Nico Rosberg, who were fifth and sixth.
The performance comparison between the cars around the lap was as expected.
Webber was more than 0.3secs faster than Alonso in the middle sector of the lap, which features a sequence of medium- and high-speed corners, while the Ferrari and the McLaren clawed back some time in the first and second sectors, which are mainly straights and slow corners.
It remains to be seen how that split will balance out in terms of overall lap time in qualifying and the race.
Hamilton said: "It was quite cool at the end, we got a couple of laps in the dry. We were able to do a couple of installation laps and work out what downforce levels to run.
"The weather was supposed to be worse today, but the weekend was supposed to be nice, but now I hear it's going to be like this all weekend.
"We seem to be OK in the wet, but I think we're also quite solid in the dry. The end of the straights will be quite interesting. Some people are very fast at the end of the straights, some people are quicker in the infield."
Hamilton survived a scary moment in the first session when his car snapped away from him heading towards the super-fast Blanchimont corner towards the end of the lap.
"It was just before Blanchimont," he said. "There was a small river across the track. I must have been doing 150mph and I had a huge tank-slapper. I'm glad I was awake to catch it otherwise it probably would have made an awful mess of the car."
HRT technical director Geoff Willis, acting as an analyst for BBC Sport, said: "I still think the Red Bulls are looking strong but Alonso and the McLarens are fighting back. It's not going to be an easy one to call, and with the DRS (overtaking aid) you'll have to fight until the last lap."
BBC 5 live analyst Anthony Davidson added: "I think the Ferraris should be pretty strong here."
DRS - which reduces drag to boost straightline speed - can be used in the race on the straight up the hill after Eau Rouge, if a driver is within a second of the car in front at a designated point on the track.
Michael Schumacher set the fastest time in the morning session as he starts the weekend that marks the 20th anniversary of his F1 debut. He and Mercedes team-mate Rosberg were the only men who set a time before it rained.
A downpour soaked the track and although it dried as the session went on, it was never dry enough for slick tyres.
The fastest driver on intermediate tyres in those conditions was Button with a lap time of two minutes 2.740 seconds - a second faster than Vettel, who was running behind him on the track.
Hamilton was fifth fastest overall, ahead of Force India's Adrian Sutil.
Unusually, Sutil sat out the second session in favour of Hulkenberg - usually reserve drivers run in the first session on Friday.
Sutil was not happy, telling BBC Sport: "We changed the session with Nico this morning. It is not good for the performance to not be in the car.
"It's a bit disappointing what the team does because it's not good for the success but I had to sit out for the afternoon and will do my best tomorrow.
"It was the only dry session we had and I missed it. But [this morning] I felt good, the car was fast and as usual we can expect a good performance here."
As drivers explored the wet conditions in the morning, Bruno Senna crashed, an inauspicious start to his first race weekend for Renault at Turn Nine.
Scot Paul di Resta, who had never driver an F1 car at Spa before, also had a similar off at that tricky downhill corner.
That incident brought out a red flag as the session was stopped while marshals recovered the Force India from the barriers.
Senna was much more impressive in the second session.
In the wet early running, the Brazilian was initially running third behind Webber and Alonso, before later moving up to second behind Vettel.
He ended the session 17th, with his Renault team-mate Vitaly Petrov down in 24th.
McLaren arrived in Belgium intending to use a new rear wing that has a more powerful straight-line speed boost when the DRS overtaking device is engaged.
But the wet conditions prevented them properly assessing its effect.
They also have a redesigned exhaust on the car in an attempt to increase downforce.
World champions Red Bull are hoping to return to the front this weekend after failing to win since Vettel's victory in Valencia at the end of June.
The team ran some bright green 'flo-viz' paint, which allows them to see the airflow over parts of the car, on the diffuser of Webber's RB7, which suggests they may have some new designs on the floor.