Nico Rosberg may have been fastest in second practice ahead of the Bahrain Grand Prix, but he remains concerned about the improving Ferraris.
Kimi Raikkonen was quickest earlier in the day and, along with team-mate Sebastian Vettel, provided the closest competition to the two Mercedes.
And Rosberg admits their pace over long spells has got him worried ahead of Sunday's race under the floodlights.
"It was a good day today," he said. "Obviously lunchtime [the first session] 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."