Jun.4 (GMM) Sebastian Vettel says questioning his credentials as a quadruple world champion is "disrespectful".

After four consecutive titles successes, culminating in the ultra-dominant second half of the 2013 season, the German has struggled even to match Red Bull newcomer Daniel Ricciardo this year with a less competitive and unreliable car.

Monaco, for example, was a particularly fraught weekend for Vettel, who was rarely confident his RB10 would even keep running.

"I have had many problems with the car," he said in an interview published on Wednesday by the German broadcaster RTL.

"So it's hard to get in your rhythm and get the best out of yourself," said Vettel, 26.

He said he is dealing with the "frustration" of 2014 by playing sports between races, taking out one of his road cars or indulging his new passion, motorcycling.

In another interview this week, with the German magazine Sport Bild, he blasted as "bullsh*t" any speculation the frustation might trigger his Red Bull exit.

Also irking him is the suggestion 2014 is showing Vettel's true colours -- not as 'great' a driver in anything other than an ultra-dominant car.

"Looking back, I only had one race without problems. That was Malaysia," he told RTL.

"Again, I think it's just important to stay true to yourself -- if the car is not working, you can be the best driver in the world, the victories will not come.

"I think if you want to make a comparison," said Vettel, "I remember when Michael (Schumacher) came back with Mercedes and the car was just not on the same level as him, maybe not at the level of his Ferrari.

"But there were many people who said 'Then Michael was just lucky when he won everything with Ferrari'. But as I said, at some point you have already proved enough to yourself and to the others."

Indeed, he hit back at his critics who suggest Vettel is not a worthy quadruple world champion, on the level of some of F1's other multiple-title greats.

"Yes," said the German, "unfortunately in our day and age it is quite normal that you come to such judgements very quickly. I think you have to learn to deal with it.

"It's very easy to say something or write something. Of course it's a pity and it's a bit like kindergarten, but sadly it's like that in Formula One. Everyone thinks everyone has something to hide."

Asked if he thinks suggestions he has 'forgotten how to drive' are even disrespectful, Vettel answered: "That's the way the world is now. In a way it is disrespectful, because these people do not know the background.

"Often they don't even want to know the background -- they want to know how controversial and not-so-great it is, not how wonderfully everything is going."