Lewis Hamilton and George Russell made an encouraging start to the 2023 test on Thursday with a car that didn't seem to suffer from porpoising like its cursed predecessor.
But the week took a wrong turn on day two, when the Brackley team struggled with the balance throughout the day in both the hot daytime and cooler evening conditions.
The team admitted it was mystified by its loss of performance and held an investigation deep into Friday night to get on top of the W14's handling issues and subsequent performance loss as it ended the day two seconds adrift.
Engineering chief Shovlin confirmed that the team managed to nail down a set-up solution that kept its car in a much better place.
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"A fair bit of work took place overnight to find some refinements to the car specification and recover our direction on the set-up," he said.
"We do seem to have made progress; both drivers felt the car was in a much better place across the range of conditions today [Saturday] and the balance is closer to what they require on a single lap and long run.
"It's clear that we still have work to do on car pace but [Saturday] has given us a much more coherent picture of where we need to focus our efforts.
"We'll be using the time ahead of next weekend to go through the data we've collected and aim to extract a bit more lap time."
Russell, who completed 83 laps in the morning session, felt the W14 "was in a much better window" in the hot conditions which proved so troublesome on Friday.
"We had a much more positive day today, we took a step forward and made progress with the issues we faced yesterday," he said.
"I believe we have improved the car over the course of testing and got the W14 in a much better window today.
"There is still plenty of work to do but we're in better shape for next week with more to come."
Hamilton took over for the afternoon and finished second on his quickest of 63 laps using Pirelli's softest C5 compound, three tenths behind Red Bull's pacesetter Sergio Perez on the slower C4s.
The seven-time world champion admitted Mercedes is "not quite where we want to be" but praised the team's progress and work ethic.
"There has been a lot of discovery and the whole team has approached it with the same mentality, working hard, not being complacent and staying focused," he said.
"We're not quite where we want to be but it's a good platform to start from. We don't know where we will be next week, but we will stay positive and continue to push to the maximum."
