Charles Leclerc and Carlos Sainz have left Ferrari atop the time sheets following Free Practice 1 at the Formula 1 Spanish Grand Prix.
The pair set the pace midway through the session, Max Verstappen third best after he encountered traffic on his fastest lap.
It was an unusually busy start as the pit lane opened with all but Lando Norris and Lance Stroll heading out immediately.
That included Juri Vips and Nyck de Vries who filled the role of young driver for Red Bull and Williams, while Robert Kubica was in Guanyu Zhou's car at Alfa Romeo Sauber.
Track conditions were near perfect, 29 degrees air temp and mid-40s track, while the chance of rain was officially zero percent.
A mixture of hard and medium compound tyres were in use, the white-walled rubber fitted to Verstappen's car when he set the early pace – a 1:23.163s.
Red Bull has only a modest list of upgrades this weekend compared to some of its rivals, with a revised front wing and floor fitted to the RB18.
At Aston Martin, Sebastian Vettel was stood in the garage watching the monitor after completing two laps early in the session, while at Haas Kevin Magnussen was also out of the car as the team managed an oil leak.
Neither had set a timed lap, nor had either Alfa Romeo Sauber driver up to the 15-minute marker of the session.
Vettel climbed back into his Aston Martin with 20 minutes having elapsed.
There had been focus on the AMR22 in the lead up to the event with the new sidepods on the car eerily similar to those on the Red Bull.
Having been investigated by the FIA, the governing body cleared the team of any suggestion of wrongdoing.
Midway through the session, Williams switched its focus to soft tyre running, with both Nicholas Latifi and de Vries running the red-walled rubber.
De Vries promptly rose to fourth with a 1:22.936s as a consequence as others began running on the softs.
Carlos Sainz was the first of the front runners to set a time on the softs, the Ferrari driver fastest of anyone through all but four microsectors around the lap to log a 1:19.907s.
George Russell then rose to second fastest, the Mercedes giving up lap time in every sector to log a 1:20.590s with 25 minutes left on the clock.
Encouragingly, there was little to no porpoising for the Brit, Mercedes having introduced a new front wing and floor, among other tweaks on the F1 W13.
Traffic in the final third of the lap on Verstappen's hot lap saw him lose some 0.4s, the Red Bull driver only second best, 1:20.164s to slot in second behind Sainz.
Next to begin a qualifying simulation was Leclerc, the Monegasque going fastest by 0.079s at the end of the lap.
Inside the final 15 minutes, Vips was given and opportunity to use a set of soft compound tyres, having been plodding around on the hards some 10s off the pace for much of the session.
The Estonian logged a 1:25.485s, suggesting his RB18 was fat with fuel as he remained rooted to the foot of the timesheets.
With five minutes remaining, the focus was firmly placed on long runs, with few if any personal best sectors being set.
As a result, Ferrari finished the session on top with Leclerc ahead of Sainz and then Verstappen, though where the Red Bull is by comparison is difficult to read given Vips was aboard Perez's car.
That the Dutchman was baulked on his flying lap also masked his ultimate pace somewhat, while Mercedes will be encouraged with their performance.
Russell was fourth best, then came Alonso for Alpine and Hamilton in the second Mercedes, that trio split by 0.2s with the former 0.7s away from Leclerc.
Both McLarens were in the top 10, Norris in seventh ahead of Pierre Gasly and then Daniel Ricciardo in ninth, with Yuki Tsunoda in the second Scuderia AlphaTauri.
Free Practice 2 is set to follow at 01:00 AEST.
Results: Formula 1 Spanish Grand Prix, Free Practice 1
Pos | Num | Driver | Team | Laps | Time | Diff |
1 | 16 | Charles Leclerc | Scuderia Ferrari | 27 | 1:19.828 | |
2 | 55 | Carlos Sainz | Scuderia Ferrari | 27 | 1:19.907 | +0.079s |
3 | 1 | Max Verstappen | Red Bull Racing | 28 | 1:20.164 | +0.336s |
4 | 63 | George Russell | Mercedes-AMG F1 Team | 28 | 1:20.590 | +0.762s |
5 | 14 | Fernando Alonso | Alpine F1 Team | 24 | 1:20.768 | +0.940s |
6 | 44 | Lewis Hamilton | Mercedes-AMG F1 Team | 27 | 1:20.811 | +0.983s |
7 | 4 | Lando Norris | McLaren F1 Team | 24 | 1:21.279 | +1.451s |
8 | 10 | Pierre Gasly | Scuderia AlphaTauri | 23 | 1:21.422 | +1.594s |
9 | 3 | Daniel Ricciardo | McLaren F1 Team | 23 | 1:21.737 | +1.909s |
10 | 22 | Yuki Tsunoda | Scuderia AlphaTauri | 28 | 1:21.814 | +1.986s |
11 | 31 | Esteban Ocon | Alpine F1 Team | 26 | 1:21.891 | +2.063s |
12 | 18 | Lance Stroll | Aston Martin F1 Team | 23 | 1:21.920 | +2.092s |
13 | 88 | Robert Kubica | Alfa Romeo F1 Team | 20 | 1:21.975 | +2.147s |
14 | 20 | Kevin Magnussen | Haas F1 Team | 16 | 1:22.089 | +2.261s |
15 | 47 | Mick Schumacher | Haas F1 Team | 21 | 1:22.146 | +2.318s |
16 | 5 | Sebastian Vettel | Aston Martin F1 Team | 20 | 1:22.164 | +2.336s |
17 | 77 | Valtteri Bottas | Alfa Romeo F1 Team | 25 | 1:22.614 | +2.786s |
18 | 45 | Nyck de Vries | Williams Racing | 28 | 1:22.920 | +3.092s |
19 | 6 | Nicholas Latifi | Williams Racing | 27 | 1:23.011 | +3.183s |
20 | 36 | Juri Vips | Red Bull Racing | 23 | 1:24.138 | +4.310s |