Valtteri Bottas has revealed Mercedes' steady first week of testing was the result of handling issues with the W10.

While the Brackley-based team typically doesn't set fast lap times pre-season it was still noticeable that both the Finn and Lewis Hamilton were some way off the pace being set by Ferrari.

In fact, though both drivers actually set marginally quicker times than the Italian team, it was only by virtue of having tried softer tyre compounds on the final day on running.

“We have work to do, but it’s a good feeling in the sense that it feels like there is potential,” Bottas told media gathered at a Petronas event in Italy on Friday.

“We do need to make improvements, we were struggling a bit over the week to find a good balance for all the corners. Some corners that were good, some corners we would have balance issues and others we would have massive balance issues."

Though Bottas insisted progress was being made by the end of the first four days, he did concede: "Some of the handling issues can only be sorted with some upgrades and we’re hoping to fix those hopefully soon.”

It is a noticeable setback for Mercedes, who had made solid progress at solving their traditional weakness of high downforce circuits last year.

The sense of urgency was also clear in the comments by Lewis Hamilton after the first test, as he called for a renewed push by the team.

“At the moment we are behind,” he said. “So, we don’t approach the first race currently at all relaxed, not that we ever do. We still have a lot of work to do.

“This is going to be the hardest year yet, but I feel I have the best group of people behind to climb that mountain together.

“We are going to face our toughest challenge but I feel like I have the right soldiers behind me.”

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Mercedes motorsport boss Toto Wolff also weighed in suggesting Ferrari could have another 1.5 seconds in their pocket and also believes the rest of the grid is closing up.

"It is pretty much everybody bunched up [behind Ferrari], which is pretty exciting between all the other teams," the Austrian claimed.

“Now it is about seeing what happens in Barcelona [test] two. I have a feeling everybody will bring stuff to the car and then again in Melbourne.

“So, probably only then will you be able to judge the competitive balance and not before.”