Nico Rosberg has described Ferrari’s long-run pace as “very worrying” ahead of a Bahrain GP that looks set to pitch Mercedes into another duel with their growing rivals.

After a less than representative Practice One at Sakhir which took place four hours before the evening start time for Sunday’s race – running subsequently labelled “useless” by Rosberg – Mercedes and Ferrari locked horns in the more relevant second session with intriguing results.

As has been the case all season, the W06 held a clear edge over a single fast lap – to the tune of half a second in P2 – but the impressive, light-on-its-tyres SF15-T closed right in on the race simulations and appeared to even be quicker on the softer of the two compounds.

That form certainly caught Rosberg’s eye, who afterwards highlighted the race pace of the Ferrari as clearly being a threat to Mercedes ahead of a race in which he is aiming to beat teammate Lewis Hamilton for the first time this season.

“Confidence is good, it was a good day today. Of course lunchtime was a bit useless because it is so hot and the race is in the evening so we don’t learn much,” he told Sky Sports F1.

“That is why this evening was very, very valuable and important to do many laps. So it has been good, but the pace of the Ferraris is very worrying. In qualifying we are again much quicker, but in the race they are quick.”

The evening race means track temperatures will be cooler than the sweltering lunchtime highs routinely experienced in the desert, and therefore not anywhere near as hot as in Malaysia last month when Ferrari beat Mercedes.

But Rosberg thinks the SF15-T may have the fundamental race pace to challenge himself and Hamilton anyway.

“Tyre-wise it’s pretty straightforward. Both tyres are quite robust and so that’s not a very interesting part of it this weekend. It’s just going to be the outright pace which is important,” the German driver added.

On the headline P2 timesheet, Rosberg’s teammate and championship leader Hamilton, meanwhile, wound up a tenth of a second shy of the sister Mercedes. The Briton, however, was generally confident with his W06 after an error on his decisive lap cost him time.

“I think I locked up into Turn Eight so lost a bit of time and there is one area which I need to work on to pick up more time, but generally the middle sector was weak,” Hamilton told Sky F1.

“But the car is feeling good.”