Valentino Rossi and Jorge Lorenzo believe Yamaha will struggle to compete among the leaders in Austria, with the latter claiming Ducati have a "huge" advantage.
MotoGP riders concluded a two-day private test at the Red Bull Ring on Wednesday ahead of next month's Austrian Grand Prix.
It is the first time the famous circuit will feature on the MotoGP calendar in almost two decades, with the riders getting their first taste of the track during the tests.
Rossi was ninth fastest after the first day and improved to fifth on day two, but the time sheets were dominated by Ducati - filling the top four spots on both days.
"It is a particular type of track," the Italian told reporters. "Very, very fast and you stay a lot of time at full throttle. There are just nine corners but sincerely speaking I like the track and it's good to ride.
"But unfortunately on the performance side it is not our track, because usually we suffer a bit with top speed and in the straight we lose quite a lot.
"We were able to improve the lap times and our position today and we're a little bit closer to the top guys. But it will be a very difficult race, especially with some of our factory rivals; they are faster than us.
Rossi's team-mate Lorenzo, who has scored just seven points out of a possible 75 in the last three races, also struggled in testing at the Red Bull Ring, finishing 11th and then sixth fastest across the two days.
"It's been hard to accept that all the Ducatis are so fast," the defending world champion said.
"It looks like this track gives them a big advantage, especially in braking stability, acceleration and top speed. They can use all their engine power here and the difference is huge.
"With the Suzuki and Honda we are not so far. But there are a lot of Ducatis and they are all very fast.
"We didn't expect such a big difference. Nothing is impossible but if we want to fight for the win in dry conditions we need to improve a lot. It looks like this track is the best one for Ducati in all the championship.
"But little by little we will try to improve and close this gap. Let's see if it's possible."