MONTMELO, Spain -- Lewis Hamilton believes Mercedes' position at the top of Formula One is under threat this year after admitting the new Ferrari looks "very, very strong" in pre-season testing so far.
Ferrari topped the timesheets at the opening two days of this week's test in Spain, while Mercedes' fastest times have been over a second off the pace.
Such a large difference in pace is likely to be down to differing fuel loads -- a quarter of a tank of fuel is worth over a second in lap time difference -- but Sebastian Vettel finished his first day in the Ferrari on Monday claiming it was close to "perfection".
"That's good for him," Hamilton said in response. "They have been looking great.
"I don't think you can put a figure on it, but Ferrari are very, very strong right now as you have seen. They are racking up great mileage as well -- it appears that they have a better package than they had last year, which means it is going to be even more of a challenge for us this year.
"For us it has just been about digging deep and trying to understand the car, so pretty much the same as the beginning of every year."
Mercedes has won 74 of the last 100 races in Formula One and all five titles since 2014, but Ferrari has closed the gap significantly in recent years. Hamilton joined the team in 2013, when it won just three races all year, but believes the upcoming 2019 season will be the most difficult since he arrived.
"I'm here for round seven with this team and it's crazy to think I've had six years with the team already," he said. "But this is going to be the most challenging year of our partnership and probably the team's time for as long as I have been with it."
But amid suggestions that Mercedes might be as much as 0.5s per lap off the pace of Ferrari on a like-for-like run, Hamilton said he was not yet panicking about the pace of the red car.
"At the moment I don't worry about anything, I just try to focus on our job," he said. "I don't know what everyone is doing, there are different fuel loads and I have been in this business a long time so I know how it goes over the first week of testing and into the second week.
"It's not a time when we need to focus on others, we let everyone else do their thing and we will really dig deep and focus on making sure our processes are better than ever before, that we are analysing the data better than ever before and that from a driver's point of view the feedback is more accurate than ever before.
"That is what we are trying to focus on and making sure we get though our run plans. We will have a better idea this time next week but even then we still won't know what fuel loads people are on and who is showing their cards and who is not."