As all eyes – and cameras – looked to Max Verstappen in his Red Bull debut, it was Ferrari's soft tyres that beat Mercedes' mediums to the 1-2 in FP1 at Barcelona.
The first team to put in any serious running on Friday morning, Mercedes grabbed the early 1-2 on the medium tyres.
However, that didn't last for long as first Kimi Raikkonen and then Sebastian Vettel edged ahead.
The Ferrari drivers, who tested a new front wing, used the soft tyres to claim the 1-2 in the opening practice session.
Vettel's 1:23.951 put him 0.138s ahead of team-mate Raikkonen.
Mercedes were next in line on the medium tyres with Rosberg 0.503s off the pace while Hamilton was a further 0.157s down.
The reigning Champions ran updated sidepod, floor and front wing as Formula 1 development war high high gear as the first race of the European leg of the championship.
Red Bull were fifth and sixth with Daniel Ricciardo less than two-tenths ahead of debutant Verstappen in what was an almost error-free session for the Dutchman. Verstappen had one notable moment early on when he out-braked himself at Turn 1 and locked up, running wide.
Williams, one of the few teams not debuting any major upgrades in Spain – Pat Symonds billing it as a "normal batch", were seventh and ninth quickest with Valtteri Bottas ahead of Felipe Massa.
The team-mates were separated by Carlos Sainz. Having missed out on a Red Bull seat, the Spaniard has a lot to prove and showed he has pace despite being in the junior team.
His best time, a 1:26.078 on the medium tyres, was 2.1s down on the P1 time. His new team-mate Daniil Kvyat was 12th fastest, half a second behind the Spaniard.
Fernando Alonso, who was running new sidepods, floor and wings on his MP4-31, completed the top ten.
1 Sebastian Vettel Ferrari 1:23.951 22 laps
2 Kimi Raikkonen Ferrari 1:24.089 +0.138 18 laps
3 Nico Rosberg Mercedes 1:24.454 0.503 34 laps
4 Lewis Hamilton Mercedes 1:24.611 0.660 33 laps
5 Daniel Ricciardo Red Bull Racing 1:25.416 1.465 25 laps
6 Max Verstappen Red Bull Racing 1:25.585 1.634 29 laps
7 Valtteri Bottas Williams 1:25.672 1.721 36 laps
8 Carlos Sainz Jr Toro Rosso 1:26.078 2.127 19 laps
9 Felipe Massa Williams 1:26.186 2.235 32 laps
10 Fernando Alonso McLaren 1:26.243 2.292 18 laps
11 Kevin Magnussen Renault F1 Team 1:26.576 2.625 30 laps
12 Daniil Kvyat Toro Rosso 1:26.583 2.632 21 laps
13 Nico Hulkenberg Force India 1:26.938 2.987 27 laps
14 Sergio Perez Force India 1:27.064 3.113 9 laps
15 Felipe Nasr Sauber 1:27.253 3.302 27 laps
16 Romain Grosjean Haas F1 Team 1:27.258 3.307 17 laps
17 Esteban Gutierrez Haas F1 Team 1:27.283 3.332 18 laps
18 Marcus Ericsson Sauber 1:27.392 3.441 24 laps
19 Jenson Button McLaren 1:27.610 3.659 19 laps
20 Pascal Wehrlein Manor Racing 1:28.084 4.133 29 laps
21 Rio Haryanto Manor Racing 1:29.052 5.101 34 laps
22 Esteban Ocon Renault F1 Team No time 6 laps
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