Lewis Hamilton set the pace in second practice at the British Grand Prix as team-mate Nico Rosberg hit trouble.Rosberg, the world championship leader, failed to do a single lap as his car suffered a water leak.
Red Bull's Daniel Ricciardo was second, 0.391 seconds off Hamilton and 0.235secs ahead of team-mate Max Verstappen.
Ferrari's Sebastian Vettel and Kimi Raikkonen were next, with McLaren's Fernando Alonso in a surprise sixth.
Tension at MercedesRosberg's problems come against a backdrop of internal tension at Mercedes following the collision between the two drivers on the final lap of last Sunday's Austrian Grand Prix.
Hamilton and Rosberg have been told they are on their "final warning" and that "financial and sporting penalties" could follow if they crash together again.
Rosberg has an 11-point advantage over Hamilton after he dropped from first to fourth following his ill-fated attempt to defend against the world champion at Turn Two at the Red Bull Ring.
Hamilton went on to win the race and Rosberg was penalised by stewards after being found at fault for the incident.
Can Red Bull catch Mercedes?Possibly. Mercedes appear to be in a league of their own over one lap, but in race trim it may be closer.
In the final part of second practice, the teams simulate the race, running heavy fuel loads.
As Rosberg sat in the garage with the gearbox off his car, Hamilton's initial pace on his first-stint race-simulation run on soft tyres was initially very strong - a second or so faster than the Red Bulls.
But as the runs progressed, the Red Bull hung on to its pace longer and by the end of the runs their average pace was very similar.
Ferrari's runs were not comparable as they ran different tyres at different times.
But BBC F1 analyst Allan McNish said Ferrari were struggling, adding: "It is Red Bull who are going to take the fight to Mercedes on today's performance."
Even McLaren might have a good British GPBehind the top three teams, Alonso's pace gave McLaren encouragement.
The Spaniard had said on Thursday he was expecting to be qualifying in the top 10 and scoring points again after three difficult races and it seems he may be on the money.
Honda engine has an upgrade on its intake system for this weekend, which has meant the use of two of the company's permitted development 'tokens'.
Team-mate Jenson Button was struggling with a lack of grip and was 0.8secs slower in ninth place despite setting his time much later in the session when the track would have been faster.
Williams's Valtteri Bottas was seventh, ahead of an encouraging showing from the new Haas team for Romain Grosjean in eighth.
Renault's Jolyon Palmer was 18th fastest, an encouraging two places and 0.349secs quicker than team-mate Kevin Magnussen, who had to sit out the first session as reserve driver Esteban Ocon was given a run in the car.
And there was another notable performance from Manor's Pascal Wehrlein, who was 17th, ahead of both Renaults, after scoring the first point of his career and for the team this season in Austria last weekend.