Lewis Hamilton admits that it looks "very close" ahead of the 2018 Bahrain Grand Prix between the Mercedes AMG Petronas Motorsport team, Scuderia Ferrari and Aston Martin Red Bull Racing.
"We had an hydraulic leak in Melbourne race and were fortunate to finish the race", Mercedes said on Twitter.
Under current regulations, each gearbox must last six consecutive races.
Raikkonen and Vettel were up front as cooler temperatures and nightfall matched the conditions for Sunday evening's race.
Speaking on Friday night after the second practice session but before his penalty was confirmed by Mercedes, Hamilton already predicted a tough fight with their biggest rivals this weekend.
"We definitely have got some work to do", Hamilton said before incurring the gearbox change. Mercedes are keen to hold on to Hamilton for the foreseeable future and the Briton is also certain he wants to remain in the sport with the team that has brought him three of his four F1 Drivers' championships. He escaped a grid penalty for Sunday's race and Ferrari was instead fined 5,000 euros ($6,100).
Both sessions on Friday were topped by different drivers, with Daniel Ricciardo in the Red Bull leading the way in the first session before Kimi Raikkonen was fastest in the second for Ferrari.
The Australian was 0.3 seconds quicker than Bottas and 0.4 seconds faster than Raikkonen in the early session. The British driver twice took his Mercedes too wide on corners. He got out and helped stewards wheel it back to the team garage.
"It was not ideal to miss so much track time". "I'll try to improve the feeling and balance to put myself in the best position possible".
The race was won by Ferrari's Sebastian Vettel, thanks to a stroke of luck provided by the deployment of the virtual safety vehicle at just the right time for the German, also a four-times champion.