Mercedes trackside engineering director Andrew Shovlin believes the team has its "work cut out" to overcome the issues that blighted its pre-season test programme.
The reigning champions emerged from the three days at the Bahrain International Circuit bottom of the lap chart after encountering numerous problems with its W12, and convinced it is behind Red Bull and other teams on pace.
In past years Mercedes has normally been reliable in testing and with a solid platform on which to build for the season, but on this occasion, there are genuine concerns.
"We've made a bit of progress with the balance on higher fuel and the car was more predictable but we can see from the data we've collected over the last few days that on race pace, we're not as quick as Red Bull," declared Shovlin.
"The lower fuel work was a more confusing picture, we didn't gain enough and we need to go and look at our approach as far too many cars were ahead of us on pace.
"We've had issues in recent years with pace in winter testing and managed to make good progress before the first race but we may have our work cut out this time.
"We've not got long before we're back here for the race so we've planned a programme of work to try and understand some of our issues and will be leaving no stone unturned in our efforts to find some more speed over the next 10 days."
After Valtteri Bottas amassed 86 laps in the morning, Hamilton followed up with just 54 in the afternoon - the second-lowest on the day - with his running notable for one 360-degree spin in the final corner as he was about to embark on a quick lap.
Hamilton recognises there are problems that need solving before the first race of the season in a fortnight's time in Bahrain.
"I've never been a massive fan of testing, I just love racing," said Hamilton. "It's probably the only year I would have asked for more days though because we've got lots of work to do.
"We've never felt in the past that we were quicker than everyone else after testing because you never know what people are doing with the fuel and setups.
"We always just focus on understanding our car the best way we can so if I'm honest, I feel the same this year. We won't know until we get to qualifying in two weeks' time exactly where we stand.
"Today we made some improvements but it's still not where we want to be. I can't pick out any one area, it just feels globally through the lap that we need to improve."
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.