Ferrari driver Kimi Raikkonen set the pace during practice at the Spanish Grand Prix, topping the time charts in the morning and afternoon sessions.
The Finn clocked 1:20.649 in the morning and, with a heavier fuel load, managed 1:21.935 in the second session.
McLaren's Lewis Hamilton was more than half a second slower than Raikkonen in both sessions.
But Renault enjoyed an encouraging day as Nelson Piquet Jr and Fernando Alonso came second and third in the afternoon.
Raikkonen's Ferrari team-mate Felipe Massa was second quickest during first practice, but finished fifth in the second.
During the first session, world champion Raikkonen, who just shaded his Brazilian team-mate, was 0.543 seconds clear of Hamilton's McLaren in third place.
Things got worse for the Briton in the afternoon as he ended 0.750 seconds down on Raikkonen, with even Force India's Adrian Sutil ahead of him in 10th.
Hamilton said: "I wouldn't say I'm more optimistic, but I wouldn't say I'm worried. I think we just have to work hard, put our heads down, and try to find the right balance.
"But it's very, very hard when you have two practice sessions when you maybe get 20 laps and you have to fight the correct balance of the car - epecially when the wind is picking up and changing."
606: DEBATE
Friday practice sessions are notoriously unreliable indicators of form, however, and Saturday's qualifying is a much more reliable indicator of form.
But the performances of Piquet Jr and double world champion Alonso hinted at an upturn in form for Renault, who have been off the pace so far this season.
They have introduced some new parts to the car for this race and are optimistic of a step forward.
Ferrari's apparent pace will be a concern for McLaren and Hamilton, who have struggled compared to their title rivals in the last two races.
McLaren have said that only after this race will a true picture emerge of the cars' relative competitiveness for the season and will be hoping they are much closer to Ferrari come race day.
Times from first practice for Spanish Grand Prix at Circuit de Catalunya:
1. Kimi Raikkonen (Fin) Ferrari 1:20.649
2. Felipe Massa (Bra) Ferrari 1:20.699
3. Lewis Hamilton (GBR) McLaren 1:21.192
4. Robert Kubica (Pol) BMW Sauber 1:21.568
5. Heikki Kovalainen (Fin) McLaren 1:21.758
6. Fernando Alonso (Spa) Renault 1:21.933
7. Nelson Piquet (Bra) Renault 1:21.936
8. David Coulthard (GBR) Red Bull 1:22.118
9. Nick Heidfeld (Ger) BMW Sauber 1:22.278
10. Jenson Button (GBR) Honda 1:22.632
11. Timo Glock (Ger) Toyota 1:23.002
12. Nico Rosberg (Ger) Williams 1:23.003
13. Mark Webber (Aus) Red Bull 1:23.015
14. Jarno Trulli (Ita) Toyota 1:23.141
15. Kazuki Nakajima (Jpn) Williams 1:23.153
16. Adrian Sutil (Ger) Force India 1:23.156
17. Giancarlo Fisichella (Ita) Force India 1:23.196
18. Rubens Barrichello (Bra) Honda 1:23.353
19. Sebastien Bourdais (Fra) Toro Rosso 1:23.952
20. Sebastian Vettel (Germany) Toro Rosso 1:24.082
21. Takuma Sato (Jpn) Super Aguri 1:24.278
22. Anthony Davidson (Britain) Super Aguri 1:25.068
Second practice:
1. Raikkonen 1:21.935
2. Piquet Jr 1:22.019
3. Alonso 1:22.032
4. Nakajima 1:22.172
5. Massa 1:22.229
6. Webber 1:22.238
7. Rosberg 1:22.266
8. Coulthard 1:22.289
9. Fisichella 1:22.383
10. Sutil 1:22.548
11. Hamilton 1:22.685
12. Kubica 1:22.788
13. Heidfeld 1:23.130
14. Trulli 1:23.224
15. Button 1:23.263
16. Kovalainen 1:23.264
17. Barrichello 1:23.415
18. Vettel 1:23.661
19. Bourdais 1:23.684
20. Glock 1:23.883
21. Sato 1:25.110
22. Davidson 1:25.163
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