Charles Leclerc set an eye-opening lap to head a Ferrari one-two in second practice at the Belgian Grand Prix.
The 21-year-old was 0.630 seconds quicker than team-mate Sebastian Vettel - and a massive 0.846secs ahead of the fastest Mercedes of Valtteri Bottas.
Championship leader Lewis Hamilton was fourth, 0.046secs behind his team-mate, but only Leclerc of the drivers in the top teams seemed to manage a clean lap.
Alex Albon was down in 10th, with Red Bull team-mate Max Verstappen sixth.
- Belgian Grand Prix second practice results
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Albon, who has been promoted from Toro Rosso for the final nine races of the season, had been right with Verstappen in the first session in the morning but ended the day 0.377secs off the Dutchman's pace.
Vettel, Hamilton and Verstappen all made errors at the final chicane on their laps. Albon's lap looked uncomfortable and while Leclerc also went into the Bus Stop chicane a little deep, he lost less time than the other top drivers.
The cars were showing wildly different performance profiles.
Ferrari were blisteringly quick on the straights, with Leclerc 0.8secs quicker than the Mercedes in the first sector, which includes the flat-out drag from La Source hairpin to the Les Combes chicane, and 0.2secs up in the final sector, including the run through Blanchimont.
But Mercedes were nearly 0.4secs quicker than Ferrari in the middle section, which includes the vast majority of the corners at Spa-Francorchamps.
However, Mercedes clearly have more pace to find, as Bottas and Hamilton were only 0.1secs quicker than the Racing Point of Sergio Perez in fifth place.
And on the race-simulation runs later in the session, Mercedes appeared to have a significant advantage not only on absolute pace, but also on tyre degradation, which was much more pronounced on the Ferrari.
Ferrari were more competitive on the medium tyre, on which Verstappen's Red Bull appeared quickest, and that may influence their decision as to how to run their qualifying strategy and therefore on which tyre to start the race.
Hamilton said his first session, in which he had two separate technical problems, "wasn't spectacular", but added: "We made some changes into second practice and it was feeling a lot better but we still have work to do. I don't know about the gaps.
"The Ferraris look quite quick. They are nearly a second quicker than us just in a straight line down to Turn Five, which is quite impressive, but we will keep chipping away at it. We are clearly in the mix.
"Ferrari have been quick on the straights all year so I had a feeling this weekend it would work really well for them. The long run was very strong. If we're quick in the race, it could be good."
Red Bull team boss Christian Horner said Albon had "done a very good job" on his first day in the car.
"Considering he has not sat in the car previously, I think he's got in, he's got a very good feel," Horner said. "He is giving feedback very consistent with Max's and he's done a pretty decent job on his first day."
Horner added that the cars were running in different specifications in the second session but that Albon was "within a few 10ths" of Verstappen on outright pace.
"He has done one day in the simulator for us and two sessions in the car," Horner said. "He said there were quite a lot of differences to the Toro Rosso that he's adjusting to, but you can tell he's very bright, he's intelligent. He's asking all the right questions, so I'd say it was a positive first day."
Perez's strong session ended in sadness, when he pulled off with what looked like an engine problem. He appeared very unhappy as he climbed out of the car - and may be fearing a grid penalty.
Behind Verstappen in sixth, Kimi Raikkonen's Alfa Romeo was seventh, ahead of Lance Stroll's Racing Point, Daniel Ricciardo's Renault and Albon.
There are a number of grid penalties caused by excessive engine changes already confirmed, largely as a result of upgraded engines being introduced by all manufacturers apart from Ferrari.
Albon, Stroll and Toro Rosso's Daniil Kvyat will start from the back and Ricciardo, his Renault team-mate Nico Hulkenberg and McLaren's Carlos Sainz have five-place penalties.