Mercedes duo Nico Rosberg and Lewis Hamilton say they are wary of the pace which Ferrari displayed during free practice for the Austrian Grand Prix.
Rosberg finished second to pace setter Sebastian Vettel, while Hamilton ended the session in fifth place.
"It's a challenge to nail the setup for this track - but I felt comfortable in the car," said Rosberg.
"The red cars are very strong here. They had very impressive pace - on higher fuel levels as well as low. They are our closest rivals, I think, but on this short track the lap times are quicker and all the cars are closer together."
Hamilton added that his off track excursions were nothing unusual but remained wary regarding Ferrari's speed.
"I had a bit of oversteer here and there but generally the car feels great and we definitely made improvements from P1 to P2," he said.
"There's just not a lot of grip out there on track and I didn't put a full lap together. I had some good sectors and was a few tenths up on a couple of laps, so the pace is there.
"I was pushing hard and learning with every lap. If you're not making mistakes you're not trying hard enough. With the tyres, the option was one step faster than the primes as usual - but other than that the difference between them wasn't spectacular in terms of feeling.
"I'm not sure how our pace looks relative to the others. I haven't seen the data but Ferrari looked quick and it's definitely closer here than at other circuits. We're always aware of the threat from our rivals and we take it seriously every race."