Lewis Hamilton stands on the brink of Formula 1 history as he has the chance this season to become the first eight-time champion.
Hamilton has succumbed in two previous title defences, with Jenson Button stealing a march with Brawn during the radical rule-change period of 2009 and then Mercedes team-mate Nico Rosberg taking a nail-biting championship in 2016.
But with Mercedes struggling for form in pre-season testing and the overhang of Max Verstappen's ruthless Abu Dhabi Grand Prix victory to close out last season, will this be Hamilton's toughest title defence to date?
Mercedes testing struggles
The biggest concern for Hamilton and Mercedes is the relative performance of the W12 compared to the Red Bull RB16B in Bahrain.
Whilst the Silver Arrows struggled to come to terms with the new rear-end aerodynamic changes, its rival was in a class of its own at the top of the standings.
This was underlined by the comments of Mercedes motorsport strategy director James Vowles after the test who said: "I would say Red Bull are ahead on performance. They are the class act from the test, but it is a test, it is not a race and it is one event out of 23.
"Are we going to have a close season? I would say so. Red Bull are a fierce adversary, they've got a strong package and clearly came out of the box very, very quick."
The problem for Mercedes wasn't necessarily outright speed as Valtteri Bottas topped the timing sheet on day two. The issue instead lies with the processes of reaching the set-up window required to achieve such times.
Other than Bottas' lap, the story for Mercedes - and Hamilton - was one of clumsy spins, low grip and reliability issues. The team must get on top of this quickly.
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