Sakhir - Lewis Hamilton will take a five-place grid penalty in Sunday's Bahrain Grand Prix following a gearbox change on his Mercedes late on Friday in another early season setback for the world champion.
The British star had wound up fourth fastest earlier in Friday's second free practice session at the Bahrain International Circuit.
"@LewisHamilton will take a five-place grid penalty for the #BahrainGP after having to change his gearbox," tweeted Mercedes.
Kimi Raikkonen was fastest as he and Ferrari team-mate Sebastian Vettel dominated the session ahead of the two Mercedes, headed by Valtteri Bottas.
Hamilton's setback in his bid to overhaul Vettel's early-season lead in the title race - following his victory in the season-opening Australian Grand Prix - was confirmed by the International Motoring Federation (FIA), the sport's ruling body.
Bottas also had a new gearbox fitted on Friday evening, but escaped a grid penalty because he had already been penalised in Australia.
Four-time world champion Hamilton, who created a minor controversy when he showed his approval on social media earlier in the day of the retention of grid girls at the Monaco Grand Prix, said before his penalty had been announced that it had been "a normal Friday".
"It's very close and we have some work to do. The tyres feel the best they have here in years, very stable with less degradation."
Mercedes technical director James Allison said after Friday practice that the team were not as competitive in relation to Ferrari as they had been in Melbourne.
"It wasn't our best day," he said. "We've got more to do overnight and in the practice tomorrow (Saturday) to five us a car that can fight in qualifying and on Sunday.
"The base for us is ok, but we don't have the edge we had in Melbourne."
Raikkonen had escaped a grid penalty of his own as he clocked a best lap time of one minute and 29.817 seconds to outpace four-time champion Vettel by 0.011 seconds.
Raikkonen, however, faced a post-race investigation into a possible unsafe release of his car, by Ferrari, following a routine pit-stop late in the session.
He pulled away, but then saw his pit crew waving their arms and pulled up at the side of the track.
It was feared he would be punished for an unsafe release - because his front right wheel was not attached properly - and given a grid penalty, but instead Ferrari were fined €5 000.
The race stewards said they felt Ferrari had done all they could to minimise the risk of the wheel coming off the car.
Bottas was happy to bounce back after his confidence-sapping crash in qualifying in Australia two weeks ago.
He said: "There is always pressure and I am treating this like a normal weekend. Here the track improves all the time, with more grip every lap.
"The car felt ok, but we need to find some more pace especially on a single lap."
Dutchman Max Verstappen, who missed the first session due to a power failure, was fifth for Red Bull ahead of team-mate Daniel Ricciardo.
Verstappen said: "We had a small issue this morning - these things happen so it doesn't concern me too much.
"I lost a lot of track time, but come race day it will depend a lot on strategy."
Nico Hulkenberg was seventh for Renault ahead of the Toro Rosso of Pierre Gasly and the two McLarens of two-time champion Fernando Alonso and Stoffel Vandoorne.