Mercedes boss Toto Wolff says his side remains “quite sceptical” about its own performance despite making its best-ever start to an Formula 1 season in 2019.

The reigning world champions have claimed four consecutive one-two finishes to kick off the campaign in style and move into an early 74-point buffer over chief rival Ferrari.

But heading to the Spanish Grand Prix, Wolff insists Mercedes is anticipating another tough weekend, with Ferrari set to introduce its first engine upgrade of the season two races earlier than originally planned.

“Our next stop takes us to Barcelona, where our opponents were blisteringly quick in winter testing, so the Spanish Grand Prix will be anything but easy,” Wolff said.

“Despite the good start to the season we remain quite sceptical of our own performance and know that there are a number of areas where we can and must improve.

“We anticipate a tough and competitive weekend, first against our rivals and also between our drivers. Everything is still very much up for grabs this season and we will be keeping our foot firmly to the floor.”

Wolff believes Mercedes’ performances across the opening four rounds of the season has not been a true reflection of the competitive order, and he insists the team cannot afford to start getting complacent.

“Nobody would have predicted four consecutive Mercedes 1-2s after winter testing,” he added.

“But here we are having scored 173 points in the first four races - a remarkable result which is first and foremost testament of the amazing work of every single member of the team. Our devotion to the task, our hunger for victory and our love of racing are stronger than ever.

“The results seem to paint a relatively clear picture, but the truth is that they're too flattering. The performance has fluctuated in the first four races. We had the fastest car in Australia and China, but we won in Bahrain thanks to our reliability when we were on the back foot in terms of speed.

“And while our race pace looked good in Azerbaijan, we were never able to build a truly comfortable gap to our rivals.

“So there is no complacency about our situation because we know that it's not truly representative of the balance of power this season so far.”