Mercedes' Nick Rosberg says his team are still holding back in testing and have not yet shown their hand to the competition.
After three days of testing, Mercedes have been no where near the top of the time sheets.
Burt Rosberg says they are confident they will be one of the team's to beat once again this season.
"We haven't shown our cards yet. We're still holding back - and for sure the car is quick," Rosberg said.
"The opposition also seems to be quick, so how quick we are nobody knows for now but we are confident."
Fastest lap times from Barcelona pre-season testing so far:
1. Nico Hulkenberg (Ger) Force India 1:23.110
2. Romain Grosjean (Fra) Haas 1:25.874
3. Kimi Raikkonen (Fin) Ferrari 1:25.977
4. Kevin Magnussen (Den) Renault 1:26.014
5. Nico Rosberg (Ger) Mercedes 1:26.084
6. Carlos Sainz (Spa) Toro Rosso 1:26.239
7. Felipe Nasr (Brz) Sauber* 1:26.392
8. Lewis Hamilton (GB) Mercedes 1:26.421
9. Daniil Kvyat (Rus) Red Bull 1:26.497
10. Felipe Massa (Brz) Williams 1:26.712
11. Jenson Button (GB) McLaren 1:26.919
12. Rio Haryanto (Idn) Manor 1:28.249