Red Bull's Max Verstappen ended Friday practice at the Belgian Grand Prix with a crash after setting the fastest time ahead of both Mercedes drivers.
The Dutchman, eight points behind Mercedes' Lewis Hamilton in the championship, lost control at the Malmedy right-hander.
Verstappen backed into the wall, causing a premature end to the session.
He said: "I just lost the rear, a bit too much oversteer to correct and unfortunately hit the wall."
- 'To see all the plastic on the beach was shocking'
- Uncertainty hangs over most intense F1 title fight for years
- Perez to stay at Red Bull for 2022
- Full practice results
Verstappen was 0.039 seconds quicker than Valtteri Bottas, with Hamilton third.
Hamilton was just 0.031secs behind his team-mate, but the Red Bull and Mercedes drivers used different tyres to set their times.
Verstappen's quickest time was on the soft-compound tyre, theoretically the quickest, whereas the Mercedes drivers' best laps were on the medium.
Both Bottas and Hamilton failed to improve when they tried the soft tyres.
Mercedes used a lower-power engine mode for their runs on the soft tyre compared with the medium, and neither driver was satisfied with the car's balance on the softer tyre.
Ferrari's Charles Leclerc also had a crash, his at the Les Combes chicane just one corner before Verstappen's incident. That ended Leclerc's session midway through, causing a brief red-flag period while the car was removed.
Leclerc carried a little too much speed into the chicane, managed to negotiate the initial right-hander, but the car ran a little wide through the following left. He lost control as he tried to moved to the other side of the track for the quick Malmedy right-hander that follows.
Leclerc said: "It has been a very tricky day overall, for everyone because of the conditions. But we have one night to work on the car and come back stronger.
"I would have preferred not to end the day in the wall, but it is part of free practice and I will try not to do these mistakes later.
"It felt quite good [in the first session] but P2 didn't feel great. We need to understand it because we did not change too much on the car."
In addition, both Alfa Romeo's Kimi Raikkonen and Alpha Tauri's Yuki Tsunoda spun at the La Source hairpin early in the first session, in which the Finn later also crashed into the pit wall on his way into the pits.
Set-up conundrums for Mercedes
Mercedes spent the day comparing different downforce levels, on a track where choosing the right aerodynamic set-up is always tricky.
The choice is between low downforce, which guarantees high straight-line speed and pace in the first and third sectors, which contain long, flat-out sections, or more downforce, sacrificing speed on the straights for pace in the sequence of demanding corners in the middle sector.
Bottas preferred the low-downforce set-up - he has a five-place grid penalty for causing the crash at the start of the last race in Hungary and needs to be able to overtake in the race. Hamilton said it was a tough choice.
"This morning we had it one way and we changed it this afternoon," he said. "Trying to find the balance is quite tricky."
Hamilton said the heavy flooding that affected the region and caused widespread damage and some fatalities earlier this summer had also changed the track, producing a nasty bump in the middle of the famous Eau Rouge swerves.
"It is a great circuit. It is very bumpy now through Eau Rouge," he added. "There is a massive bump right at the compression point which we've never had before. It is really sharp. You feel it on the backside. I think it was something to do with the mudslides, the rain."
Verstappen, who was running the higher downforce set-up on the Red Bull, said he was happy with the car.
"It was good," he added. "The whole day we were quite happy, a few things to fine-tune, but a very positive start."
The forecast of mixed conditions for the weekend, and the chance of a wet race in which higher downforce would be preferred, further complicates the choice.
Behind the top three, Alpine's Fernando Alonso was fourth fastest, ahead of Alpha Tauri's Pierre Gasly, Aston Martin's Lance Stroll and the second Alpine of Esteban Ocon.
McLaren's Lando Norris was ninth fastest behind Aston Martin's Sebastian Vettel and ahead of Ferrari's Carlos Sainz.
Williams driver George Russell was 17th.
- Canine crazy? Hayley Pearce finds out what has got us barking mad for puppies since lockdown
- Young, Welsh and Bossin' It: How have these young entrepreneurs made profit during a pandemic?