Lewis Hamilton's hopes of winning the Bahrain Grand Prix have taken a major dent after he was hit with a five-place grid penalty.

The defending champion was bidding to get his title charge back on track this weekend after losing out to Ferrari's Sebastian Vettel at the season opener in Australia a fortnight ago.

But he will now be bumped down the order following an unscheduled gearbox change on his Mercedes.

It means Hamilton will start only sixth even if he manages to post the quickest time in qualifying on Saturday.

Hamilton's Mercedes team revealed that the British driver suffered a hydraulics leak at the curtain raiser in Melbourne, adding that he was even fortunate to finish the race.

Hamilton had been poised to win at Albert Park only for a timing error on the Mercedes pit wall during a virtual safety car period to gift-wrap the victory to his rival Sebastian Vettel.

Even before news broke of Hamilton's penalty here late into the Bahrain night, the 33-year-old Englishman already appeared to be playing catch-up.

Hamilton was only fifth and fourth respectively in Friday's two practice sessions with Ferrari's Kimi Raikkonen setting the overall pace. Vettel was second for Ferrari with the Italian team seeming to hold the advantage over Mercedes in the hotter temperatures.

Speaking before his grid penalty was revealed, Hamilton said: "It looks very close between all the teams and we definitely have got some work to do to try and see if we can eke out to be ahead of the others.

"Practice has shown that it's incredibly close between the three teams. It will be a tough weekend."

Hamilton's team-mate Valtteri Bottas will also take on a new gearbox, but - under the sport's rules - avoids a penalty as he is permitted a free change after his qualifying crash in Australia.

Raikkonen and Vettel finished half-a-second clear of their Mercedes rivals in practice, and Vettel, while pleased with his team's progress, expressed caution over Friday's times.

"It looked as though we were a bit quicker than the rest but at this point I wouldn't draw any conclusions," Vettel said.