Lewis Hamilton was well off the pace as Mercedes were jolted by their "poor start" to testing ahead of the new Formula One season. (More Sports News)
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."
Not the first day of #F1Testing any of us were hoping for. Debriefing what went down on day one in Bahrain with @ValtteriBottas pic.twitter.com/oPLxdX5hXS— Mercedes-AMG PETRONAS F1 Team (@MercedesAMGF1) March 12, 2021
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.