LEWIS HAMILTON may have finished top of the timesheet in practice for Sunday's British Grand Prix but he was left to ponder the latest failure of his Mercedes.

Two retirements this season - in the opening race in Australia and again in Canada last month - have cost Hamilton dear in his battle for the Formula One world title with team-mate Nico Rosberg.

The 29-year-old heads into his home race 29 points adrift of Rosberg and in dire need of a result to quell the German's forward momentum after a run of three races in which he has heavily outscored the Briton.

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But at a time when Hamilton was beginning to flex his muscles around Silverstone by posting the fastest lap on the quicker of Pirelli's two tyre compounds for this weekend - the medium - his car ground to a halt with 30 of the 90 minutes remaining.

The deep, collective groan could be heard from the majority of the 70,000-strong crowd at the Northamptonshire venue as Hamilton clambered out of the cockpit of a car that had stopped between turns three and four - The Loop and Aintree respectively. It was proof again Mercedes are far from bulletproof in a season they have so far dominated, although Hamilton must be wondering 'why me?' as Rosberg has avoided such issues.

The 29-year-old has so far finished in the top two in all eight races to eke out his margin over Hamilton, although he did suffer the same loss of ERS (energy recovery system) in Montreal.

However, whereas Rosberg managed to nurse his car to the runner-up spot, Hamilton's failure resulted in a loss of the rear brakes, and with it retirement. At least there will be some satisfaction in finishing 0.228 seconds ahead of Rosberg at the end of the two Friday sessions, with his best lap a one minute 34.508secs.

Again, as in FP1, only Fernando Alonso finished within a second of the dominant Mercedes duo, with the Ferrari star 0.736secs adrift of Hamilton. Red Bull pairing Daniel Ricciardo and Sebastian Vettel were fourth and fifth quickest respectively, just over a second off the pace, with Williams' Valtteri Bottas 1.5secs down in sixth.

Vettel is refusing to get stressed by a campaign that is proving to be one of his most difficult. Renault power unit problems have resulted in three retirements for the reigning four-times world champion this year, as many as he has suffered in the previous three seasons combined.

"Currently we don't have the car to win unless something out of the ordinary happens because at the moment Mercedes are in a position to win every race," he admitted. "That doesn't really matter because you still try to get the maximum out of your package. It's part of the game, You have good times, bad times, but right now I'm not stressing."