Lewis Hamilton returned to the top of the timesheet as Mercedes dominated first practice for the Spanish GP with a heavily-revised car.
All eyes had been on what Red Bull would introduce for Barcelona after a disappointing start to the season, but it was Constructors' Championship leaders Mercedes whose W08 made the bigger immediate visual impact with significant changes to the front of the car.
What's new on the Mercedes in Spain?
After an off-the-pace weekend in Russia, Hamilton hit the front early at Barcelona with a fastest time of 1:21.521. He was only 0.029s quicker than Sochi race winner Valtteri Bottas, but nearly a second up on Ferrari as Kimi Raikkonen and Sebastian Vettel finished third and fourth respectively.
Vettel, who has a 13-point championship lead, lost track time early in the session when he stopped in the exit of the pit lane. The German initially suspected a potentially troublesome gearbox, but Ferrari later put the issue down to a broken wheel.
But while Vettel was able to return to the circuit, his old title rival Fernando Alonso's session was ended inside one lap after an oil leak in his McLaren's Honda engine triggered yet another failure and sent the Spaniard, the Barcelona crowd's local hero, spinning off the circuit.
After returning to the paddock, Alonso's manager confirmed to Sky F1 his driver had left the circuit to return to his hotel - with the Spaniard soon posting a picture of him playing tennis with the message "keeping the body active".
He is expected to return in time for Practice Two, which begins at 1pm UK time.
Alonso had warned McLaren on Thursday he would be forced to look elsewhere for 2018 if their partnership with Honda did not start showing signs of a front-running potential heading towards next year. Amid a succession of reliability problems, Alonso has yet to record a race finish this season.
Meanwhile, Red Bull are targeting a more immediate return to the front and, as expected, their unveiled a heavily-upgraded RB13 on Friday morning.
"With Red Bull, who arguably have most to catch up, it's a good upgrade package," reported Sky F1's Ted Kravitz. "This is what they should have started the season with. So what we are seeing on the detail in terms of front wing, bargeboards, engine cover, diffuser, really just puts Red Bull at the cutting edge of what everyone has got at the moment.
"It's not some amazing leap forward in terms of some aerodynamic pieces we've never seen the like of in the pit lane, it's just getting up to where they should have started the season."
On the timesheet, Max Verstappen and Daniel Ricciardo finished close to the pace of the Ferraris, although both drivers lost track time at the start and end of the session respectively.
Haas made a positive start to the weekend with the seventh and eighth quickest times, although Kevin Magnussen ran off the road in the closing stages.with mechanical problems.
Spanish GP Practice One Timesheet
1. Lewis Hamilton, Mercedes, 1:21.521
2. Valtteri Bottas, Mercedes, +0.029
3. Kimi Raikkonen, Ferrari, +0.935
4. Sebastian Vettel, Ferrari, +1.079
5. Max Verstappen, Red Bull, +1.185
6. Daniel Ricciardo, Red Bull, +1.563
7. Kevin Magnussen, Haas, +2.149
8. Romain Grosjean, Haas, +2.237
9. Nico Hulkenberg, Renault, +2.472
10. Carlos Sainz, Toro Rosso, +2.483
11. Sergio Perez, Force India, +2.667
12. Esteban Ocon, Force India, +2.803
13. Stoffel Vandoorne, McLaren, +2.879
14. Felipe Massa, Williams, +3.097
15. Daniil Kvyat, Toro Rosso, +3.121
16. Marcus Ericsson, Sauber, +3.445
17. Pascal Wehrlein, Sauber, +3.661
18. Lance Stroll, Williams, +4.398
19. Sergey Sirotkin, Renault, +4.772
20. Fernando Alonso, McLaren, No time
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