Lewis Hamilton will need to get back on pole in Spain

Lewis Hamilton is back on top of the world championship as Formula One returns to Europe but the Mercedes driver faces a big battle to stay ahead of Ferrari's Sebastian Vettel in Sunday's Spanish Grand Prix.

Vettel is going for his fourth pole position in a row, and at a circuit that should be a proper benchmark of performance after a thrilling and sometimes chaotic first four races of the season.

The German has never before started from the top slot in Barcelona, however, whereas Hamilton -- with more poles than anyone in the history of the sport (73) -- has done so in three of the last four years and won twice.

Pole matters because only three of the last 17 races at the Circuit de Catalunya, albeit also three of the last seven, have been won from anywhere else on the grid.

Past performance may provide some comfort to Hamilton but he knows his four-point advantage over fellow four times world champion Vettel owes a lot to luck and that Ferrari have true pace.

"There's two weird races which have kept us within the mix but you can't rely on those for the 17 or however many are left," Hamilton said after being gifted the win in Azerbaijan two weeks ago.

"We need ultimate performance and confidence in the car," added the reigning champion.

"I've got the pace within me, the car has got the pace within it but we're not unlocking it...we've definitely got to improve in lots of areas."

Barcelona, resurfaced with smoother asphalt but holding no surprises after weeks of pre-season testing there, could come as something of a relief to Vettel after two chaotic races off the podium following two wins.

It is also where the technical battle between the teams, now within easy reach of their factories and enjoying the first use of their palatial paddock 'motorhomes' this season, really starts to heat up.

"It's the first step of the development race that looks like it could run to the very end of the season," commented Mercedes team boss Toto Wolff.

"It will be interesting to see how much performance the teams can find with their upgrades and what impact those upgrades will have on the team ranking -- both at the front and in the midfield."

Spain is also no stranger to drama, with the two Mercedes drivers colliding in 2016 in a race that saw Dutch teenager Max Verstappen win on his Red Bull debut.

Verstappen arrives this time after being cast in the doghouse along with Australian team mate Daniel Ricciardo for a collision in Baku.

Barcelona, with good memories for both, offers a chance to hit the reset button.

"We have many updates coming, hopefully they will be positive and we can be even closer to the front, but of course everybody else will also bring new parts," said Verstappen, whose team mate won in China.

"It could be quite a defining moment for the season and I’m interested to see how everyone will perform."