Sebastian Vettel told to improve his performances by Red Bull adviser

Vettel has achieved just two podium finished in seven grands prix this season as he struggles to compete with Mercedes duo Lewis Hamilton and Nico Rosberg.

The 26-year-old has also regularly raised concerns regarding the quality of Red Bull's car as new regulations have enabled Mercedes to dominate the early stages of the campaign.

Vettel's complaints have largely centred around reliability issues - he has twice been forced to retire - but Marko has told the German to focus on his own form, rather than other issues.

"I told him: 'There are 50 engineers working on this engine and they can't sort it out," Marko told BBC Sport. "'You're a top driver, it's up to you. You have to do it'.

"If he can't sort out the car, the easiest thing is the driver. He has to raise his game."

Vettel sits some 80 points behind driver's championship leader Rosberg, with the next race coming in Austria next weekend.