Charles Leclerc spearheaded a Ferrari one-two in first practice for the Spanish Grand Prix as the F1 teams paraded their raft of upgrades at Barcelona's Circuit de Catalunya.
Leclerc posted a lap of one minute 19.828secs in the SF-75 that for the first time this year is sporting a major upgrade that includes a new floor body, along with a new rear corner and rear wing.
The championship-leading Monégasque finished 0.079s ahead of team-mate Carlos Sainz, seeking to end a run of incidents in recent races that have blighted his season so far.
Red Bull's reigning champion Max Verstappen, who has cut what was a 46-point deficit to Leclerc to just 19 points over the last two races, was third quickest, a third of a second adrift.
Verstappen was on course to claim top spot at one stage after setting the fastest times in the first two sectors before encountering heavy traffic that compromised him in the final sector and cast him behind the two Ferraris.
As for Mercedes duo George Russell and Lewis Hamilton, the Britons were fourth and sixth on the timesheet, sandwiching Alpine's Fernando Alonso.
The W13 has proven to be a troublesome car so far given the level of porpoising experienced by both drivers.
Sporting a new floor body, rear corner, floor edge and front-wing endplate, Russell and Hamilton appeared to bounce far less than at previous events.
However, the car seemingly lacked pace in comparison to Ferrari and Red Bull, with Russell 0.762s down and Hamilton a further two-tenths of a second back.
McLaren has introduced the biggest upgrade package for this weekend, with a total of 10 components updated, including the front and rear wings, floor and sidepods.
Lando Norris and Daniel Ricciardo were seventh and ninth quickest at a circuit left baking in 30-degree Celsius heat, leading to a track temperature of 46.
Norris, though, was 1.451s down and Ricciardo 1.909s off the pace, with Pierre Gasly in his AlphaTauri splitting the pair, and with team-mate Yuki Tsunoda completing the top 10.
In a heavily revised Aston Martin, which has been cleared of being a copy of Red Bull with its latest upgrades, Lance Stroll and Sebastian Vettel were 12th and 16th, the latter 2.336s behind Leclerc.
In a practice outing for Alfa Romeo, Robert Kubica was 13th quickest, six-tenths up on team-mate Valtteri Bottas as the Pole sat in for Zhou Guanyu.
On his F1 practice debut with Williams, Mercedes reserve and Formula E champion Nyck de Vries was quicker than Nicholas Latifi by a tenth of a second.
Red Bull reserve Juri Vips, standing in for Sergio Perez, ran more of a hard-tyre test programme with the RB18 and propped up the timesheet, 4.310s down.
Spanish Grand Prix first practice results
1. Charles Leclerc [Ferrari] 1:19.828s
2. Carlos Sainz [Ferrari] +0.079
3. Max Verstappen [Red Bull] +0.336
4. George Russell [Mercedes] +0.762
5. Fernando Alonso [Alpine] +0.940
6. Lewis Hamilton [Mercedes] +0.983
7. Lando Norris [McLaren] +1.451
8. Pierre Gasly [AlphaTauri] +1.594
9. Daniel Ricciardo [McLaren] +1.909
10. Yuki Tsunoda [AlphaTauri] +1.986
11. Esteban Ocon [Alpine] +2.063
12. Lance Stroll [Aston Martin] +2.092
13. Robert Kubica [Alfa Romeo] +2.147
14. Kevin Magnussen [Haas] +2.261
15. Mick Schumacher [Haas] +2.318
16. Sebastian Vettel [Aston Martin] +2.336
17. Valtteri Bottas [Alfa Romeo] +2.786
18. Nyck de Vries [Williams] +3.092
19. Nicholas Latifi [Williams] +3.183
20. Juri Vips [Red Bull] +4.310