Barcelona - Sebastian Vettel insisted on Thursday that Ferrari's team spirit remained strong despite rival Mercedes reeling off an unprecedented four straight season-opening one-twos.

The four-time champion German admitted that the Italian outfit had struggled in the opening races after looking so strong in pre-season testing at the Circuit de Catalunya.

But he said he expected to see the car and team bounce back to their best on returning to the track for this weekend's Spanish Grand Prix.

"The car was really good in testing," he told reporters at a news conference.

"Then we arrived in Australia and struggled to feel the same and the first four races have been up and down.

"Deep down, we know the car is strong. And the spirit is as good or better than last year.

"We're in good shape and we're confident about the parts we bring here," he added, referring to a new upgraded power unit and other upgrades to be introduced this weekend.

"This weekend will be interesting for us because we had a good feeling here not so long ago - so it will be interesting to see how it behaves this weekend.

"I'm confident if we can get to the level, we will be very competitive."