Formula One's sole three-day test before the new season concluded in Bahrain on Sunday.

Here, the PA news agency dissects the key themes as Lewis Hamilton prepares to open his championship defence on March 28.

– Are Hamilton and Mercedes in trouble?

Standing under the lights of the Sakhir paddock, Hamilton delivered 10 words that painted an accurate, albeit surprising, picture ahead of his bid for an eighth world title. "It appears that we're not quick enough at the moment," he said.

Some might argue that it has been said before, and that Mercedes, the sport's all-conquering team for the last seven years, are yet to show their aces. Both assertions are probably correct. But what is also true, is that the three days of testing were Mercedes' poorest of any pre-season campaign in recent memory.

Hamilton had two major spins – one mistake from the Briton is rare, let alone two – they ran into mechanical trouble, they finished bottom on the lap count, and they failed to trouble the top of the time charts in any of the six sessions.

Andrew Shovlin, Mercedes' trackside engineering director, went further than Hamilton when he concluded: "From the data we've collected over the last few days, on race pace, we're not as quick as Red Bull. 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."

– What's gone wrong?

The Silver Arrows won 13 of the 17 races last year en route to claiming a seventh drivers' and constructors' double – and with the make-up of last year's machines carried over into 2021, Hamilton had been expected to cruise to title number eight. Of course, he still might, but Mercedes are under no illusions that, as it stands, Red Bull are quicker than them.

Enforced rule changes to the floors of this year's cars have decreased downforce, and while the new-for-2021 Pirelli tyres are more durable, they are heavier and less grippy than their predecessors.

Both Hamilton and team-mate Valtteri Bottas struggled with the handling of the car, and the world champions now have less than a fortnight to determine why.

– So, it is looking good for Verstappen and Red Bull?

There was an air of smugness as Verstappen sat sandwiched between Hamilton and Bottas for a press conference on the concluding day of testing. As the Mercedes drivers were grilled over the team's indifferent pre-season form, Verstappen did not knock back Red Bull's tag as favourites. "It is too early to say, but I have positive feelings in the car," he said.

Red Bull have been notoriously slow-starters, but the evidence of testing suggests they have come out the traps with greater intent in 2021. Verstappen posted the fastest time of the week and in the Dutchman, they have a driver who can take the fight to Hamilton.

On the other side of the garage, Sergio Perez, hired to replace Alexander Albon, brings much-needed experience to the former world champions. As Hamilton said: "Red Bull will be a different machine, a different animal this year."

– Could we see Hamilton and Verstappen going for the title?

That is the dream ticket for Formula One and its new CEO, Stefano Domenicali. Despite both Hamilton and Mercedes' recent brilliance, the sport is crying out for an end to the British superstar's dominance.

Hamilton, 36, and Verstappen, 23, are a class above the rest of the field, and the prospect of the two dicing for the title is a mouth-watering one.

We should expect Mercedes to recover, but the major question heading into the new season is this: how hard can Red Bull push the world champions over a record-breaking 23-round campaign?

– And what about the rest?

McLaren, fresh from finishing third in last year's constructors' championship, were suitably encouraged by their testing campaign. Powered by Mercedes this year, and with new signing Daniel Ricciardo slotting in alongside Lando Norris, the British team are optimistic of another promising year.

Red Bull's sister outfit, AlphaTauri also looked strong, while double world champion Fernando Alonso impressed on his return to action with Alpine.

But there are big question marks over both Aston Martin and Ferrari. Four-time world champion Sebastian Vettel never looked comfortable in his new machine, and he hit reliability problems, too. Ferrari, who have admitted their attention is already on 2022, appear to have made little to no improvement on last year's lacklustre challenger.