Nico Rosberg has admitted his Mercedes team had not shown their hand so far in pre-season testing in Barcelona.
The German and team-mate Lewis Hamilton have done more laps than anyone but not come close to setting the fastest times on any of the days so far.
But Rosberg said: "We haven't shown our cards yet. We're still holding back - and for sure the car is quick.
"The opposition also seems to be quick, so how quick we are nobody knows for now but we are confident."
Mercedes have done more than a third more laps than any other team over three days at Spain's Circuit de Barcelona Catalunya, but Rosberg and Hamilton were only fifth and eighth fastest in the list of times on Wednesday.
They were in similar positions on the first two days.
Rosberg, 30, said Mercedes were mainly concentrating on reliability at this stage of testing.
"We need to push the car to its limit in order to find little problems," he said.
"At some point something will break and for now we need to keep on going to find out where the little weak points are.
"Lap time-wise, of course, it's not realistic where we are."
Who was the fastest?
The fastest time of the day was set by Force India driver Nico Hulkenberg, who used the super-soft tyre to lap in one minute 23.110 seconds.
That is the second fastest time of the week, 0.2secs down on the lap Ferrari's Sebastian Vettel did on the new ultra-soft tyre on Tuesday.
It was also half a second quicker than Vettel managed on the super-soft on Tuesday.
Vettel handed the Ferrari over to team-mate Kimi Raikkonen on Wednesday and the Finn was third fastest, 2.8secs slower than Hulkenberg, using the medium tyre.
Second fastest, on soft tyres, was Romain Grosjean for the new US-based Haas team, who are entering F1 this season with significant support from Ferrari.
It was the continuation of an impressive debut for the team, whose other driver is Mexican Esteban Gutierrez.
Kevin Magnussen, who was drafted into the Renault team only last month after they dropped Pastor Maldonado, was fourth fastest, despite stopping out on track three times with reliability problems.
Jenson Button was 11th fastest in the McLaren-Honda, his running ended three hours early with a hydraulic leak that caused a fire.
He was ahead only of novice Rio Haryanto, the Indonesian spinning at turn five on his first day driving an F1 car.
Headline lap times from pre-season testing are notoriously poor indicators of real form.
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
Hulkenberg set fastest time on super-soft tyre; Magnussen, Grosjean and Nasr on the soft; all others on medium.