Red Bull advisor Helmut Marko has suggested that the defending champions are viewing Lotus as its main contenders for honours following the opening four rounds of the new F1 season.

Sebastian Vettel currently tops the drivers' championship on the back of two wins from the first four races, with Lotus' Kimi Raikkonen in second spot - the Finn having finished second to Vettel in Bahrain and having won in Australia at the start of the year.

Lotus are also second in the constructors' championship and Marko said it was no surprise to Red Bull to see the Enstone-based team right in contention at the head of the field.

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"We've known for a long time that they will be maybe the toughest opponents," he told Kleine Zeitung. "At the moment we fear Lotus more than we do Ferrari.

"But there's only been four races, and we know that things can change. Actually, what is making the Lotus so quick in the races is a mystery to us."

Lotus driver Raikkonen has been linked with a Red Bull race seat for next year, although Marko refused to comment on the speculation.

"I've said it several times," he said. "We make our driver decisions before August, and that will be the case this year, too."