Nico Rosberg says he is 'worried' that Ferrari will pose a significant challenge in the Bahrain Grand Prix after the team posted competitive long run pace in free practice.

The German ended the first day of free practice action with the fastest time, emerging a tenth ahead of Mercedes team-mate Lewis Hamilton and half a second clear of the leading Ferrari of Kimi Raikkonen in third position.

However, it is Ferrari's form over multiple laps in FP2 that has given Rosberg reason for concern, with analysis showing Sebastian Vettel proving notably quicker than his countryman at a consistent rate of half a second a lap.

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With this in mind, though Rosberg is confident he has the measure over a single lap, he admits he is 'worried' about the challenge of his rivals come race day.

"It was a good day today, obviously lunchtime was a bit useless because it's so hot and the race is in the evening, so we didn't learn much and that's why this evening when the temperature went down again it was important to do many laps.

"It was good but the pace of the Ferraris is very worrying because in qualifying we are again much quicker, but in the race they are quick.

"In the evening we learned that we were very quick on one lap again and the Ferrari is very dangerous to us on the long run because they have serious pace."