Silverstone - Sebastian Vettel warned Ferrari on Friday his team needs to take a 'significant step' if they are to mount a challenge to Mercedes at this weekend's British Grand Prix.
The four-time world champion finished fourth behind pace-setting Valtteri Bottas, the Finn's Mercedes team-mate Lewis Hamilton and third-placed Charles Leclerc in the second Ferrari.
"I'm not entirely happy," said Vettel.
"It was a tricky afternoon. We tried a couple of things.
"Now we need to see what of the stuff we tried we keep and what we dismiss. And, hopefully, tomorrow we'll take a significant step.
"Mercedes looked very, very fast and are the ones to beat so we'll see where we are tomorrow. The car was quite nervous," he added.
Frenchman Leclerc, who was faster than Vettel in both the morning and afternoon sessions, said Mercedes were emphatically faster than Ferrari.
"To be honest, on my side, I expected Mercedes to be the team to beat this weekend and they confirmed that today - that they are very, very quick in race pace," he said.
"We need to work. We struggled a lot on race runs and a bit less in qualifying runs, but we still have to find pace," he added.