Look at the onboard video footage from the fastest laps set by Sebastian Vettel and Hamilton during preseason testing and you will see two laps separated by 0.003s. However, the video shows so much more than the stopwatch as Vettel effortlessly eases his car from corner to corner while Hamilton -- who ranked 21st in ESPN's World Fame 100 -- uses every inch of the track and corrects snaps of oversteer at high speed.

The times might have been close -- for all-important loads might also have been close -- but Vettel was undoubtedly the more comfortable driver over the duration of the lap, and that counts for an awful lot when it comes to maximising the potential of a race car.

Of course, Ferrari has started the season in a position of strength before. Vettel has won the past two Australian Grands Prix and still gone on to lose both championships by the end of the year. So, a good start to a 21-race season is no guarantee of a drivers' title by the end of it, but Mercedes' difficult start comes with all the hallmarks of a previously dominant team struggling to maintain its position at the top of the pile after a regulation change.

The history of Formula One is littered with winning streaks being brought to an end by revisions to F1's rulebook. A change to the tyres hobbled Ferrari in 2005; McLaren and Ferrari were both knocked into the midfield by a significant aero change in 2009; and a new engine formula in 2014 stopped Red Bull from claiming a fifth consecutive title. Mercedes' hegemony survived the last major rule change between the 2016 and 2017 seasons, but in that time Ferrari closed the gap significantly -- and arguably had a car capable of championship success during both years.

Under last year's regulations, Mercedes had a finely honed machine, and it could be that it has lost more ground than the rest of its rivals as a result of the rules reset. The simplification of the front wing, brake ducts and barge boards under the 2019 rules might not be as big a visual change as the wider cars of 2017, but the differences have still had noticeable impact on the aerodynamics of each car.

Could 2019 be a defining season for Lewis Hamilton? Mirco Lazzari gp/Getty Images

Ever since 2009's rule change, teams have been using the front wing to manage the aerodynamic wake generated by the front wheels and force it out sideways. Those two rotating lumps of rubber create a nasty cocktail of turbulent air, and it has the potential to play havoc with the downforce-generating devices further downstream. Last year, the turbulent air could be forced out sideways to allow a cleaner flow to the floor and rear of the car, but in 2019 the most effective ways of conditioning that air have been eliminated by the new regulations.

The key this season is finding a way to claw back control of the flow structures that were so carefully controlled by the old front wing and a way to continue to separate the front wheel wake from the rest of the car. With just testing to look at, it's still too early to tell whether Mercedes has succeeded or failed in that task, but the early signs are that the car is lacking some of the rear stability the drivers need to push to the limit. Hamilton will have been aware of that the moment he first drove the car in anger, and it seems his worst fears have been backed up by the data Mercedes collected in Barcelona.

But the challenge facing the world champions is not insurmountable. There were moments last year when Ferrari clearly had the quicker car and Mercedes was able to react by upping its own development rate. Hamilton is aware of his team's capability, and he is experienced enough to know the importance of talking it up in the media.

"It [Ferrari's performance advantage] doesn't make it easier, that's for sure -- naturally it makes it harder," he said in the second week of testing. "Already last year there were many, many occasions where we as a team were behind on performance so we had to overachieve on delivery. Now we just have to reach even further than we did last year when we were behind.

"Last year, we weren't behind anywhere near as much as we are now, I would say, so now we have to reach even further. That means we have to squeeze our performance even more, but we're going to have to be careful also in that scenario because that can just push you over the edge and you can have faults. But I've got the best team around. We've got experience. It's no coincidence that we're world champions, so we just have to be diligent and stay balanced."

Lewis Hamilton during day four of F1 Winter Testing at Circuit de Catalunya, Barcelona. Eric Alonso/MB Media/Getty Images

For a neutral fan, the idea of seeing Hamilton in a less-than-competitive car is tantalising. Anyone who has tuned in to the sport in the past five years will have witnessed Hamilton winning from the front -- at times making it look ludicrously easy. He had tight battles with former teammate Nico Rosberg between 2014 and 2016, but the fight with Ferrari this year promises to be another step entirely.

Unlike the battle with Rosberg, he won't have the same equipment as his closest rival, and that means he won't be able to rely on his natural talent to make the difference when it matters. If the gap to Ferrari is as big as Hamilton is suggesting -- at 0.5s per lap -- it has the potential to cause huge frustration when the perfect race weekend is not rewarded with the victory it deserves. It's not clear how he will react to that, but watching it will be fascinating.

"As a driver, for sure, I've got to figure out how I can pull more out of myself," he added. "But currently I don't have an answer for that."

Come this weekend's Australian Grand Prix, it is likely the gap won't be as big as Hamilton feared in testing. In fact, it's possible that, on a different track and in race conditions, there will be very little to choose between the Mercedes and the Ferrari. But if Hamilton is even half right, he is going to face one of the biggest challenges of his career. If he overcomes it, it could be the defining season that marks him out as the greatest racing driver of all time.