Red Bull enters the start of the final Formula One test session in Bahrain on Thursday anxious to make up the ground lost to rivals previously.

“Mercedes is a big step in front of us. We must have worries about Australia (Grand Prix on March 16) and probably beyond,” Red Bull advisor Helmut Marko told Bild on Thursday.

So far Red Bull have managed considerably fewer laps in testing than other teams, especially German challengers Mercedes.

Power issues have been frequent and Marko added: “Renault have to get the engine in control.” But the engine has not been the only difficulty and frustrations are adding up for world champion driver Sebastian Vettel.

Marko said Vettel was “’not amused’ that we (Red Bull) are so far behind. But being angry doesn’t bring anything.”

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