Lewis Hamilton has admitted his current form will not be enough to win the world championship despite the Mercedes driver holding the lead of the title race.

Hamilton, the reigning world champion, leads Ferrari rival Sebastian Vettel by four points after victory in a topsy-turvy Azerbaijan GP last week propelled him to the top of the standings.

But Hamilton is under no illusions about the precariousness of his position at the summit having struggled to keep pace with Vettel and Ferrari since the season-opening Australian GP.

"At the moment l am punching below my weight - and that is not sufficient to win a world championship," Hamilton admitted ahead of this weekend's Spanish GP.

"I have zero comfortability. Since the last race l have not thought for one second that l am leading the world championship.

"At the moment we are capitalising on surrounding circumstances that have hindered the outcome of a race. I'll take it for now but in the long term l can't continue to rely on it."

Barcelona, a known quantity to all the teams given its unofficial status as the sport's winter residence but also a track which provides a tough all-round test of an F1 car, is generally considered to provide a barometer for what the rest of the season may hold.

Hamilton added: "If this weekend we start to get on top of the issues we have had and we are more consistent then l will be a lot happier."

Ferrari, on the other hand, have seemingly encountered few difficulties dialling in their 2018 car and Vettel was this week identified as the championship favourite by Sky F1 pundit Damon Hill.

The Ferrari driver has claimed pole in the last three events and topped the timesheets at the Circuit de Catalunya when F1 was last in Barcelona two months ago for winter testing.

"So far we can be happy," said Vettel. "We have been very competitive and have had a chance to fight for the win in most of the races."

Acknowledging the proximity of Red Bull to Mercedes and Ferrari on pace, Vettel continued: "It is very tight at the top and small things can make the difference. But it's nice that whether you start first or sixth, you can fight for the win in terms of pure performance - which is good for people watching too because you don't know what will happen."

Hamilton focused on understanding 2018 tyres

Hamilton has pinpointed understanding the new-for-2018 Pirelli tyres as potentially holding the key to his championship aspirations.

"The problem is that the tyres are shifting from session to session," the Mercedes driver said. "We change the balance of the car but it doesn't correlate

"It's like a moving target which we are struggling to steady. That's really what we are working on.

Mercedes are bringing new car upgrades this weekend, although that will be true of most if not all the teams this weekend following F1's return to Europe.

"We were very good here in winter testing and we have definitely improved since then. But Red Bull and Ferrari made a bigger gain than us from testing to Melbourne so we expect again a very close fight. But we left testing with a good set-up and that should help us."

Vettel, meanwhile, is ready to play the long game in what will be the joint-longest season in F1 history.

"There are so many races to go that it's pointless to talk about the fight for the championship," he cautioned. "Right now, there is a fight to maximise the result in every race.

"It's pretty straightforward for a long time. If you can get the best result every weekend you are racing that will bring you to a good position to fight for the championship but that is not until October."

But the significance of Sunday's race should not be underestimated. Mercedes territory for the last five years, a win for Ferrari in Barcelona will surely be interpreted as a clear sign that their ten-year title drought may be about to end.

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