Valentino Rossi's progress stalled on the second night of practice for this weekend's season-opening MotoGP round in Qatar.

The seven time MotoGP champion, who will make his much-anticipated Ducati race debut on Sunday, began the extended weekend-schedule well with fifth in Thursday night's opening session.

But the Italian lapped slower in Friday night's second practice, before making only a modest 0.173sec improvement over his Thursday time in the third and final session later in the night.

That left the #46 in eighth place, 1.271sec from Repsol Honda's Casey Stoner, who has led all three free practice sessions this weekend.

Stoner reduced his lap time by 0.717sec between FP1 and FP3.

"Today was a bit difficult, because we had hoped to make a bigger improvement, but in the end we didn't' lower our time enough," admitted Rossi.

"First of all, my shoulder bothered me a little. I wasn't as strong as I was yesterday, and in the end of the session, I could feel it. Also, we still don't know this bike perfectly and, as happened to us in the test, we get to a point where it's difficult to make a step forward with the setup.

"It's partly a matter of experience. We're trying to work with the weight distribution, especially on the rear, but we still haven't found the right balance.

"We're missing half a second with our pace to be in the group where we hope to be Sunday [0.5sec would put Rossi within reach of third place Andrea Dovizioso].

"Anyway, today we tried two setup solutions that were quite different, and we chose one that we'll try to refine tomorrow."

Aspar's Hector Barbera was once again the top Ducati rider on Friday night, as he had been (with the help of a tow) on Thursday. Barbera was 0.04sec quicker than Rossi in FP3.

"Yesterday we finished third fastest and after improving slightly today we're sixth, which isn't bad at all," said the Spaniard, preparing to start his second MotoGP season. "There is still a long way to go in this grand prix but we have started the season well and these results are a confidence boost.

"I tried a long run on a soft tyre but it didn't work well and it was on the harder one that I improved at the end. I am unsure about tyre choice at the moment because I am comfortable on both. At the end of the session I saw Alvaro [Bautista's] crash and I just want to take this opportunity to wish him all the best."

Rossi's factory team-mate Nicky Hayden was just 14th quickest out of the 17 riders.

"Things didn't really go a lot better today. My position is worse than yesterday, and my lap time is hardly any better," said Hayden. "I'm still having trouble with the bike in the long corners, but honestly, a rider should be able to find a bit more time than I have.

"After two days of testing and the practice sessions, my time isn't that much better than it was in the test. It's a bit frustrating to be so far back for the first race, but the team and I will try to figure it out."

Qualifying in Qatar takes place on Saturday night.

Rossi won last year's Qatar Grand Prix for Yamaha.