Valtteri Bottas clocked in fastest for Mercedes at the end of opening practice for this weekend's Belgian Grand Prix at Spa, the Finn pipping Red Bull's Max Verstappen late in the session by 0.164s.
Pierre Gasly was third in the dry running, the AlphaTauri driver followed by Ferrari chargers Charles Leclerc and Carlos Sainz.
Lewis Hamilton was a lowly P18 in the session, but the championship leader who was testing a different aero configuration was on course for a spot among the top three when he backed out of his fastest lap.
At the risk of stating the obvious, it wouldn't be Spa without some rain. And the sporadic pre-session showers incited the field to take its time before it headed out on track at the start of the weekend's first practice.
The Ferrari's of Leclerc and Sainz were the first to venture out onto the now largely Ardennes circuit, with the pair followed by a flurry of cars, but installation laps remained the plan.
About fifteen minutes into the session, Lando Norris put the first time on the board, the Briton powering his medium tyre shod McLaren around in 1m50.191s.
Alfa's Kimi Raikkonen indulged in a harmless spin at La Source, as did AlphaTauri's Yuki Tsunoda, as both drivers exceeded the track's early grip level on the exit of the low-speed hairpin.
Up ahead, several drivers took turns lowering the benchmark as everyone gradually dialed in some speed. Thirty minutes in, looking at the top five, Verstappen stood at the top of the timesheets with a 1m46.879s, the Red Bull charger followed by Sainz, Esteban Ocon, PGasly and Daniel Ricciardo.
Meanwhile, Bottas and Hamilton were down in P11 and P13 respectively as Mercedes appeared to focus on aero work, sampling different downforce settings on their cars.
Further down the order, Raikkonen suffered another on-track set back when the Finn clouted the wall on the entry to the pitlane, a clumsy mishap that required some minor repairs in the Alfa garage.
In the Williams camp, Nicholas Latifi popped up to P2 with 20 minutes left on the clock, but Aston Martin's Sebastian Vettel, Ricciardo, George Russell and Lance Stroll overhauled the Canadian in short order as they made the most of Pirelli's soft compound.
With teams facing the prospect of rain this afternoon, everyone was eager to maximize their dry running in the session.
Getting down to business, a soft-shod Bottas went fastest, lowering the benchmark to 1m45.199s. Hamilton, running a higher-downforce rear wing compared to his teammate, realized a purple sector 2 before backing out of his lap when he stumbled upon Latifi.
Meanwhile, Leclerc came perilously close to sticking it into the barriers when he ran wide at Turn 7 and tested the new gravel trap run-off area.
With just minutes to go, rain appeared in the pitlane, encouraging most drivers to wisely head for the pitlane, which left positions unchanged.
Bottas thus topped the session, the Finn beating Verstappen by 0.164s, with Gasly, Leclerc and Sainz rounding off the top five. Thereafter followed Perez, Vettel, Norris, Ocon and Fernando Alonso, with 1.413s separating the top ten.
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