Carlos Sainz emerged fastest from a problematic first practice session for the Belgian Grand Prix which will have a mixed-up grid on Sunday due to a raft of power unit penalties.
Sainz spearheaded a Ferrari one-two, leading the way around the Spa-Francorchamps circuit with a lap of one minute 46.538secs, pipping team-mate Charles Leclerc by 0.069s.
The session was overshadowed to some degree by news of the plethora of power unit and gearbox penalties affecting Red Bull's Max Verstappen and Leclerc.
Both will be in a battle to avoid the back row of the grid as McLaren's Lando Norris, Esteban Ocon in his Alpine, Mick Schumacher's Haas and Valtteri Bottas for Alfa Romeo are also in contention.
Verstappen ultimately finished third quickest overall, 0.217s adrift, with Mercedes' George Russell 0.858s down in fourth place.
The hour-long run was hit by the weather, however. The rare soaring temperatures of 30 degrees Celsius on Thursday gave way to the kind of conditions normally associated with Spa ahead of FP1.
Grey cloud and drizzle dominated throughout late morning and early afternoon but relented in time for the drivers to take to a relatively dry circuit.
The session was cut short, though, by 14 minutes due to Kevin Magnussen's Haas stopping on track, just out of La Source, due to a power unit problem.
Given the safety aspects surrounding such a stoppage, the car had to be handled with care before being craned onto the back of a flat-bed truck.
With nine minutes remaining, it sparked a late flurry from the remaining 19 drivers but the rain soon returned to prevent most from setting a representative time on the soft tyres.
Leclerc, however, was the big mover in slotting in behind Sainz and ahead of Verstappen.
Lewis Hamilton had to settle for ninth on the timesheet, although his lap of 1:48.420s was on the medium tyres.
The seven-time champion had just embarked on a soft-tyre outing when Magnussen brought the session to a halt, and he opted not to return once it restarted.
Behind Russell, was Aston Martin's Lance Stroll, while Alex Albon also delivered a fine lap in his Williams to clock 1:47.835s, with the Thai-British driver followed by the man in the news this week in Daniel Ricciardo.
The Australian, who will leave McLaren at the end of the season with his contract terminated a year early, was 1.543s behind Sainz in seventh on the timesheet ahead of AlphaTauri's Yuki Tsuonda, Hamilton and Red Bull's Sergio Perez in 10th.
Liam Lawson, given an outing for AlphaTauri with Pierre Gasly stepping aside, was 19th, 5.527s adrift.
Not for the first time this season, Bottas encountered a problematic FP1, managing just two untimed laps that left him at the bottom of the timesheet.
Belgian Grand Prix first practice results
1. Carlos Sainz [Ferrari] 1:46.538s
2. Charles Leclerc [Ferrari] +0.069
3. Max Verstappen [Red Bull] +0.217
4.George Russell [Mercedes] +0.858
5. Lance Stroll [Aston Martin] +0.899
6. Alexander Albon [Williams] +1.297
7. Daniel Ricciardo [McLaren] +1.543
8. Yuki Tsunoda [AlphaTauri] +1.772
9. Lewis Hamilton [Mercedes] +1.882
10. Sergio Perez [Red Bull] +1.936
11. Nicholas Latifi [Williams] +1.947
12. Zhou Guanyu [Alfa Romeo] +2.134
13. Lando Norris [McLaren] +2.932
14. Fernando Alonso [Alpine] +3.126
15. Sebastian Vettel [Aston Martin] +3.275
16. Esteban Ocon [Alpine] +3.777
17. Kevin Magnussen [Haas] +4.444
18. Mick Schumacher [Haas] +4.721
19. Liam Lawson [AlphaTauri] +5.527
20. Valtteri Bottas [Alfa Romeo] no time