The Mexican, who had definitely been off the pace on Friday, returned to Spa and led the way over defending champion Max Verstappen, while Carlos Sainz led the Scuderia attack in third.
The Aston Martins were the first cars to hit the track, with both drivers wearing a set of mediums tires to get ahead of Lewis Hamilton and Sergio Perez – the pair taking on the soft red-walled Pirellis.
Hamilton was the first to complete a lap, but circulated seven seconds off the final pace to exit Alfa Romeo’s double act as the first to post something like a representative lap time.
After seven minutes of the hour, with a new set of C4 softs, Zhou Guanyu propelled himself to the top of the standings thanks to a 1m47.795s effort.
That put him just over a tenth faster than his teammate Valtteri Bottas. The Finn was the fastest driver in all sectors one and two, but lost in the latter part of the lap.
Anyway, the Alfas were initially clear for 2.2 seconds after 10 minutes – aided by Verstappen blocking the front right in La Source before complaining about the all-round tire overheating.
After 10 minutes, only the Ferrari and Haas cars were left to emerge – the Scuderia was probably working to save a set of tires after an ill-fated strategy betting on hard tires in Hungary.
With Hamilton struggling in ninth after a massive bout of upset by the Bus Stop chicane, before Daniel Ricciardo did the same, the Aston Martins stormed into third and fourth, led by Lance Stroll.
Kevin Magnussen finally emerged after 21 minutes, the Dane had an electrical shutdown in FP1, but when he came out the other Haas was called back to the pits – the team had noticed a problem for Mick Schumacher after just six laps of running.
But Magnussen was right into the rhythm, finishing third on a set of soft tires to only be two tenths behind Zhou thanks to the fastest run of everyone through S2 at the time.
It would be another four minutes before the Ferraris finally took to the track, and Sainz quickly reached the top of the charts thanks to his qualifying simulation of 1m46,461s on soft tyres.
Carlos Sainz, Ferrari F1-75
Photo by: Erik Junius
Leclerc was four tenths slower than his teammate after the first attempt, the Monegask received a hefty grid penalty for engine and gearbox changes along with Lando Norris, Schumacher, Zhou and Verstappen.
Red Bull reacted when Perez surrendered a speed of 1m45.972s to run half a second faster, but Verstappen would remain only 12th after being locked in the Bus Stop chicane.
He then withdrew completely under instruction, having supposedly missed a yellow flag for Pierre Gasly’s AlphaTauri running alongside Blanchimont.
The defending champion was to be investigated for this, but was acquitted before the end of the session. This interruption meant that Verstappen had to run again after setting the fastest time of anyone through the second sector – the twisty run from Les Combes to Stavelot.
So, with 20 minutes to go, Verstappen put together an improved run to go fastest over his teammate by 0.5s and a full 1s lead over Sainz as the Red Bull RB18s came to the fore.
Ferrari’s retaliation was then ruined by a crash of Leclerc to raise a red flag.
His F1-75 popped at the corner to Les Fagnes and he spun across the gravel to push against the outer tire wall – although he seemed to escape with no apparent damage.
Despite Leclerc resuming shortly afterwards, the red flag was called into action very quickly to effect a three-minute stop.
When the session resumed for a final shootout, Verstappen improved to a 1m45.184s across the line thanks to his work in sectors two and three.
But Perez’s superior last-ditch run through the early part of the lap meant he took over the FP3 boast with a 1m45.047s and took a tenth lead over Sainz.
Norris finished fourth ahead of Fernando Alonso, while Mercedes’ best comeback George Russell was sixth, as Leclerc was limited to seventh.
Behind Sebastian Vettel in eighth – the AMR22 slowed on the cool-down lap and the powertrain went into limp mode before restarting – Esteban Ocon raced to ninth.
However, the Alpine racer complained that Sainz held him up by Eau Rouge to ruin an improved lap. Gasly clocked 10th ahead of Ricciardo, Hamilton and the two low-downforce Williams – Nicholas Latifi who gave Alex Albon the fastest run of the session through the fast first sector.
F1 Belgian GP – FP3 Results