The German manufacturer enjoyed an encouraging first day of testing, but things went awry on Friday as it struggled to find a good balance with the W14.
George Russell ended the day down in 13th on the timesheets, two seconds off pacesetter Guanyu Zhou, while Lewis Hamilton was a further three tenths adrift.
Mercedes’ efforts were further hampered by a hydraulic problem that hit Russell late in the afternoon. The issue left him stranded out on track and cut short the team's running.
Mercedes has admitted to being left confused about why things turned around so strangely during the second day of action, and said there was some urgency to get answers before the final day begins.
Trackside engineering director Andrew Shovlin, who was seen in deep discussion with team boss Toto Wolff after the session, said: “We've not had a strong second day; stopping on track with a reliability issue wasn't great and we have struggled to get the car balanced well across the changing conditions.
“We've got some investigations going on to understand why this has been such a challenge today when yesterday it was fairly straightforward. That work will continue into the night and no doubt we'll understand more come the morning.
“It will be interesting tomorrow to see if we can understand the drop in performance and mitigate the lost track time.”
Speaking earlier in the day, Wolff told F1.TV that the W14 had not been behaving as well as the team and drivers would have liked.
“The car is out of balance this morning,” he said. “And you can see in the driving, leaving [tyre] marks on acceleration. It's hot and we just didn't find the right set-up for these conditions, which is part of the learning, I guess, with a new car.”
Asked if the situation was a concern, Wolff said: “Yeah, it definitely is because it's not the driver who's overdriving the tyres or pushing it, it's the car that doesn't give him enough grip from the rear. So this is something we need to sort out over the course of the journey.”
Russell said that the team was working hard to find answers in the telemetry about what had gone wrong.
“Even though we didn't complete our full programme, we uncovered some interesting things in the data throughout the day,” he said. “That is a positive and we will analyse these overnight with the aim of finding lap time.
“We haven't unlocked everything yet in the W14 and everyone is working hard to do so and maximise our final day of testing tomorrow.”