Lewis Hamilton was well off the pace as Mercedes were jolted by their "poor start" to testing ahead of the new Formula One season.
Gearbox trouble meant Valtteri Bottas could only complete six laps in the morning in Bahrain, before team-mate Hamilton had a clearer run and ran through 42 laps in the afternoon.
However, reigning drivers' champion Hamilton was only 10th-fastest on the day, with Red Bull's Max Verstappen topping the time sheets, while Bottas was the slowest of all the 17 drivers involved.
Mercedes' trackside engineering director Andrew Shovlin declared: "It's been a poor start to the season from us.
"Once we got running it was clear we didn't have the car in the right balance window and, while we made some progress during the day, we're not happy with how the car is performing and it's pretty clear we have some work to do."
The Mercedes W12 is the car with which the team hope to land an eighth consecutive constructors' title, while Hamilton will be chasing a fifth successive drivers' championship and a record eighth overall.
Hamilton said: "Today definitely wasn't the cleanest of days that we've had, but these things are sent to try us.
"Everyone has worked extremely hard to get us here. We had that mishap in the morning which was very unfortunate for Valtteri in terms of losing time on track and the team is working hard to try and understand what that was.
"I think they did a great job to turn the car around. The second session wasn't without its challenges, but we exist to find solutions to the problems we're faced with."
Hamilton was affected by a sandstorm and said Mercedes would look to "take the positives" from their shaky day, with two more days of testing to come.
Positives were not immediately obvious, and Bottas said: "In recent years, things have been pretty much bulletproof so it's not a familiar experience."
However, the Finn added on the team's website: "I am confident we can recover and I’m looking forward to two good days over the weekend."
As well as testing, Bahrain will host the opening grand prix of the Formula One season, with that race scheduled for March 28.
Perez says media lack understanding of Red Bull issues
Sergio Perez accused the media of lacking understanding regarding Red Bull's recent issues, as the team rediscovered their pace in Friday's first practice at the Azerbaijan Grand Prix.
Red Bull looked set to dominate Formula One again after making a flying start to 2024, but McLaren have made it a fight for the constructors' championship crown.
Max Verstappen won seven of the first 10 races of the season but has since failed to record a victory in six outings, while Perez has gone 11 races without a podium finish after achieving four in the first five races of the year.
Red Bull rediscovered their pace at first practice in Baku on Friday, though, with Verstappen finishing fastest ahead of Lewis Hamilton and Perez following in third.
Speaking ahead of that session, Perez acknowledged he had suffered more than Verstappen from difficulties with the team's RB20 car, but said many onlookers lacked understanding.
"I've had them probably for the last 15 months, with the car development, the direction, the trajectory we've been taking," Perez said of his issues.
"I think it's a thing with the style of Max. The good thing is he was less sensitive to the issues we had, just by his nature.
"Some people don't understand it, especially from the media side, that different driver styles can come to different conclusions.
"Ten, 12 races ago, I was complaining about these issues, but now they are so big that even Max is feeling them. I think the solution will benefit me even more with my style.
"It's very clear in the data, the team is fully aware of it and hopefully we can fix the issues soon going forwards. Then we can unlock a lot of performance from the car."
Friday's opening session at the Baku street circuit – where Perez is the only driver with multiple victories – saw Verstappen set a leading time of 1:45:546 on his last lap.
A chaotic session saw three red flags waved, with Charles Leclerc causing one of them when he hit a wall at Turn 15.
Williams rookie Franco Colapinto also hit a barrier less than 10 minutes after the session resumed, with the running also being paused to allow the removal of debris from a dusty surface.
McLaren's Lando Norris had the fourth-fastest time, with Carlos Sainz in fifth and Oscar Piastri sixth, ahead of Fernando Alonso and George Russell.
Norris targets drivers' championship win by 'fighting and battling' Verstappen
Lando Norris wants to try and topple Max Verstappen in the drivers' championship on merit, despite McLaren confirming they will prioritise the Briton over Oscar Piastri.
Norris, who is currently 62 points behind Verstappen ahead of the Azerbaijan Grand Prix this weekend, is looking to bounce back from his second-place finish in Monza.
The Dutchman has not won in his last six Grand Prix's, which is his worst run since the end of the 2020 season, but Norris has only closed the gap to him by seven points in the standings.
McLaren's team principal Andrea Stella revealed the team would "bias" their operations towards Norris as they seek to win the drivers' and constructors' titles this season.
"I don't want to be given a championship," Norris started.
"Yes, it would be great to have a championship, and in the short term you feel amazing, but I don't think you'd be proud of that in the long run.
"It's not how I want to win a championship."
Norris has shown his ability to challenge the three-time world champion on track this season, with the Briton finishing on the podium 10 times so far.
The pair also locked horns at the Austrian Grand Prix, with Norris and Verstappen colliding on lap 64, forcing the former to retire in what could prove to be a pivotal moment in deciding who claims the drivers' championship.
Norris needs to gain an average of just under eight points a weekend over the remaining eight Grand Prix's to beat Verstappen to the title but wants to do it the right way.
"I want to win it by fighting against Max and beating Max, beating my competitors and proving I'm the best on track. That's how I want to race," Norris said.
But both Norris and Piastri played down the idea the Australian might be asked to sacrifice a win for his team-mate.
Norris said: "Probably not. In general, probably for lower positions, but if he's fought for a win and he's deserving of a win, then he deserves to win.
"Oscar is still fighting for his own racing, he's still going out and doing his stuff. And it could be that there's no time this year that he needs to help me.
"It's more that I've got Oscar's help when needs be, but he's still going out with that intent of every session of fighting for himself."
"The team have asked me to help out, and I've said for the last few races that if I was asked, then I would," Piastri added.
"Of course, naturally, as a driver, it's never an easy thing or a simple thing to agree to. But again, there's a much bigger picture in play than just myself.
"It still needs some more discussions, but the main point is it's not just going to be me pulling over for Lando every race because that's how none of us, including Lando, want to race.
"Trying to go through all the scenarios is impossible. We don't want to discuss that publicly, but the main one is if we feel one has done a much better job on a weekend, whichever way it is, we want that person to be rewarded.
"And that's where it becomes a little bit tricky still."
Azerbaijan Grand Prix: Piastri asked to back Norris title bid by McLaren
McLaren are set to prioritise Lando Norris over team-mate Oscar Piastri in their bid to win both Formula One championship titles, starting in Baku this weekend.
McLaren currently sit second in the constructors' championship, eight points behind Red Bull, with both Norris and Piastri also in the top five of the drivers' standings.
Having tossed and turned about how to attack the final eight races of the season, the British-based team have opted to back Norris ahead of the Australian.
However, Andrea Stella said the decision to back the Brit in dethroning Max Verstappen would only happen within the team's principles of sportsmanship and fairness.
"The overall concept is we are incredibly determined to win, but we want to win in the right way," Stella said.
Norris arrives at the Azerbaijan Grand Prix off the back of a third-placed finish in Monza, coming in six seconds behind team-mate Piastri.
But Norris was able to add 16 points to his drivers' championship tally, closing the gap to Verstappen to 62 points after the Dutchman finished in sixth.
Piastri finds himself 106 points behind Verstappen, but Stella continued to reiterate McLaren's 'papaya rules'.
"We [will] bias our support to Lando but we want to do it without too much compromise on our principles," he said.
"Our principles are that the team interest always comes first. Sportsmanship for us is important in the overall way we go racing.
"And then we want to be fair to both drivers."
Until now, McLaren had allowed Norris and Piastri to race each other without interference from the team.
But a series of meetings at McLaren have resulted in a shift in policy after failing to capitalise on a one-two in qualifying at the Italian Grand Prix last time out.
Norris, who started on pole, endured another slow start out of the blocks, allowing Piastri to narrowly squeeze past his team-mate on the opening lap.
But in doing so, it allowed Ferrari's Charles Leclerc to position himself between the two McLarens, with the Monegasque eventually roaring to victory on Italian soil.
"What we don't want to see any more is a situation like in Monza in which we enter a chicane P1/P2, and we exit P1/P3, because that is a detriment to the team," Stella said.
"The team interests come first and these are the situations that above all we need to fix, because eventually, as a matter of fact, the way we entered the race in Monza left the door open for this situation.
"After Monza, three objectives: we need to make sure that anything that happens on track is not to the detriment of the team.
"Second objective, how do we win both championships, both drivers committed to help? But what we don't want to do is win in a reckless way.
"Those are the three topics and they define the way we go racing in Baku. This will be updated after Baku."
Verstappen triumphed in seven of the first 10 races of the 2024 season, with the Dutchman looking destined to run away with the drivers' championship once again.
But Red Bull's recent struggles on the track have been alarming, with Verstappen now six grands prix without a victory (his worst streak since the penultimate GP of 2020 when he went 11 in a row).
It has allowed Mercedes, Ferrari and McLaren to catch up in the respective championship standings, with Red Bull left bemused as to why a once-dominant car has seen its performance drop so dramatically.
But another team's misfortune has certainly been McLaren's gain, with Stella assuring that both Norris and Piastri were onboard with the shift in philosophy.
"The conversations have been very collaborative," Stella concluded.
"Even when I said to Oscar: 'Would you be available to give up a victory?' He said: 'It's painful, but if it's the right thing to do now, I will do it'.
"Every driver is hard-wired to go for a victory. So I am always very impressed by the level of team spirit and maturity and collaboration that we found in this period."
DRIVERS TO WATCH
Lando Norris - McLaren
With all the talk surrounding Norris ahead of this weekend's race in Baku, the Brit will be keen to make amends and continue his pursuit of Verstappen.
For the first time in 2024, Norris has finished ahead of the Dutchman in two straight races. Throughout Verstappen's dry spell, the Brit has only closed the gap on him by seven points in the drivers' championship, though.
While Norris' full focus will be on closing the gap to Verstappen, he has the opportunity to put himself among the elite British racers to have competed in the competition.
The 24-year-old is one podium away from equalling Stirling Moss and John Surtees to enter the top 10 of British drivers with the most podiums (24).
If he does achieve that feat, David Coulthard (62) and Eddie Irvine (26) will be the only British non-world champions who have achieved more podiums than Norris in the history of F1.
But there is also a lot at stake for McLaren. They are now just eight points behind Red Bull in the constructors' standings, knowing a better finish than their opponents will see them top the rankings for the first time since Brazil in 2005.
And they have reason for optimism too. McLaren have seen one or both of their drivers stand on the podium in each of the last 12 races in the competition.
Should they do so again in Baku, it will equal the team's second-best run in the history of the competition (13, which it recorded on two other occasions, between the United States and Portugal in 1990, and Germany 2011 and China 2012), only behind a sequence of 19 between Australia 2007 and Malaysia 2008.
Sergio Perez - Red Bull
No one saw Red Bull's recent struggles coming after a dominant start to the season, but they could spring a surprise with a driver who relishes the Baku street circuit.
Sergio Perez is still waiting to add to his five wins he has managed with the Austrian-based team, but he may be confident of adding to that tally in Azerbaijan.
This weekend's grand prix will be the circuit's seventh appearance in the F1 calendar, with the Mexican the only driver to prevail twice around the Baku track (2021 and 2023).
In fact, it is the only grand prix where the Mexican has won more than one race in his career.
Given his impressive form in Azerbaijan, Perez has also scored the most points (100) in Baku, the only track on which he has reached the 100-point mark.
Only a Lewis Hamilton victory and fastest lap would see the seven-time world champion overtake Perez this time around (75 points for Hamilton).
However, the Mexican has now gone 10 consecutive grands prix without finishing in the top five of the standings, the same number of times as in his previous 41 races in the competition.
CHAMPIONSHIP STANDINGS
Drivers
Max Verstappen (Red Bull) - 303
Lando Norris (McLaren) - 241
Charles Leclerc (Ferrari) - 217
Oscar Piastri (McLaren) - 197
Carlos Sainz (Ferrari) - 184
Constructors
Red Bull - 446
McLaren - 438
Ferrari - 407
Mercedes - 292
Aston Martin - 74
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