Lewis Hamilton scored an important psychological blow ahead of the British Grand Prix by beating Nico Rosberg in second practice.
The Mercedes cars were in a league of their own as ever and Hamilton was 0.228 seconds clear of his team-mate.
But Hamilton failed to do a race-simulation run after his engine suffered an oil-pressure problem as he left the pits.
"The car felt good. It was just the most important run I missed," he said.
"It happens. I don't know why it happens to my car so much. I don't really know what the car feels like for the race but I will try to get some long runs tomorrow."
Hamilton said he could look at Rosberg's data, but not having experienced the feel of the car out on the track on heavy fuel was a set-back.
He said: "The long-run data [from your team-mate] doesn't help you because we drive differently in terms of how the tyres are lasting, whether you have to shift the balance, or the brake balance or whether you have to lift and coast. It's OK, I'll be all right."
The Briton is 29 points behind Mercedes team-mate Nico Rosberg in this season's dramatic title race, despite having won more races. Hamilton has retired from two grands prix, whereas Rosberg has not finished outside of the top two so far.
The pair resume their battle in qualifying tomorrow, which is live on BBC Two, radio 5 live and online.
Behind the Mercedes cars, Ferrari's Fernando Alonso was third fastest ahead of Red Bull's Daniel Ricciardo and Sebastian Vettel.
Valtteri Bottas, in whose Williams an engine failure restricted Susie Wolff to one flying lap in the first session, was sixth.
The Finn's team-mate Felipe Massa, who suffered a heavy crash on the exit of Stowe corner in the first session, was 11th fastest after his team repaired his car in time to get him out about 10 minutes into the session.
McLaren's Jenson Button and Kevin Magnussen were seventh and eighth ahead of Ferrari's Kimi Raikkonen, who was again struggling compared to team-mate Alonso and was 1.31secs behind the Spaniard.
Although Alonso was the next fastest man after the Mercedes, Red Bull's Daniel Ricciardo may well in fact be the silver cars' closest challenger.
His fastest time was set on the harder tyre, on which he split the two Mercedes drivers, but he failed to produce a good lap on the faster 'medium' tyre.
Even so, the Australian ended the session less than 0.3secs behind the Ferrari, and 0.116secs quicker than Vettel, who did do a lap on the 'medium'.
Red Bull team principal Christian Horner said: "I think we've underperformed, Daniel did an amazing lap to split the Mercedes on the harder tyre and didn't improve - in fact went a hundredth slower - on the softer tyre and we think it should be worth 0.6-0.7secs."
Ricciardo did not do a race-simulation run on heavy fuel later in the session because of a turbo problem and a similar fate befell Hamilton, whose engine stopped as he headed out of the pits.
BBC F1 analyst Allan McNish said: "Clearly Hamilton has not had the best preparation; stopping as he went out for his race simulation run is a bit of a handicap. But the Mercedes has lap time in hand.
"Ferrari were best of the rest but Red Bull may be able to pull it out in qualifying.
"One team I am surprised about is McLaren. It might be because it's hotter but to be seventh and eighth is a decent showing for them."