Valtteri Bottas was left puzzled after his Mercedes became "undriveable" during his final run of second practice ahead of the Bahrain Grand Prix.
The Finnish driver completed a long run on a set of medium-compound tyres at the end of the season-opening Friday at the Bahrain International Circuit.
At one stage, Bottas had four consecutive lap times deleted for exceeding track limits at turn four prior to delivering his damning verdicts over the team radio, saying the feeling of the car was "terrible" before declaring it "undriveable" as the chequered flag dropped.
After finishing fifth in the session behind Red Bull's Max Verstappen, courtesy of his qualifying simulation run, Bottas explained his feelings following the final run of the day.
"The last long run we did was pretty inconsistent and we couldn't really put any laps together," said Bottas.
"The car, as I said [over the team radio], was undrivable at times. It is a big puzzle but it's a Friday and that's why we practice."
Team principal Toto Wolff provided an answer as to why the balance of the W12 was so poor in the latter stages. He said: "I think we left him out there a lot with the tyre so it was pretty undriveable towards the end."
Much of the narrative taken from winter testing was that Mercedes, with its balance issues, could be under immediate threat from Red Bull.
After a week and a half to analyse the problems that arose from the three-day test at the same venue, Bottas at least suggested the car had improved ahead of Sunday's opening race.
"We are definitely in the mix," he claimed. "McLaren looked really strong and Red Bull was as expected. I think we are there. I don't think we are the fastest at the moment but we are not far off the target of where we want to be.
"It felt better than it did in testing, the feeling of the car felt better. It feels like we are still not there yet in terms of car balance.
"At least it was a trouble-free day with no big issues but the main thing in my mind is that we still have plenty of work to do if we want to fight for the pole and for the win."
GPFans is a multi-platform, multi-language brand dedicated to Formula One coverage. We bring you all the ins and outs of the sport, 24/7, everything from up-to-the-minute news and features to the latest viral stories and clips.
We believe that a new generation of exciting, outspoken drivers will make F1 more popular than ever before, and we want to give our users access to as much of their heroes as possible, on and off the track. From Lewis Hamilton to Max Verstappen, Daniel Ricciardo to Sebastian Vettel, we provide in-depth analysis of every every Grand Prix in the season, from Australia to Abu Dhabi.
With Formula One under the new ownership of Liberty Media, how the sport is being covered is evolving, and GPFans will look to be at the heart of this progression into new media, as one of the fastest-growing sites covering the king of motorsports.