Casey Stoner made life a little uncomfortable for Ducati's 2016 MotoGP line-up by being the quickest Desmosedici rider on the final day of the Sepang test.

Off a Ducati MotoGP bike since 2010, the retired double champion proved he had lost none of his natural talent by quickly adapting to an all-new machine, tyres and electronics.

Yet the Australian again declared he has doesn't plan on entering a race and likened his new test role to that of an engineer.

"Honestly, I have no plan to race," Stoner said. "My role is completely changed now. I'm not an engineer, but I'm taking the role of an engineer as a test rider, because a lot of test riders have careers where they want to achieve things and move forward, but I'm interested in each step, what it does, why it does it.

"I really want to see this [project] go further forward and take it to another level. So I'm going to start learning all I can from these guys to get a better understanding, because there's a lot of complicated systems.

"If I wasn't going to race years ago, something that I wanted to do is get an engineering degree. So it would be nice to learn a lot more from these guys [at Ducati]."

After a private test on Saturday, Stoner lined up alongside the MotoGP regulars for the first time since 2012 during Tuesday's second day of official testing.

While ninth on the timesheets, the withdraw of the soft rear tyre after Loris Baz's accident masked his true pace and a closer examination revealed Stoner was in the top five on the harder rear tyre - and quickest Ducati.

That was confirmed on Wednesday, when all riders used the hard tyre throughout the rain-interrupted final day. Stoner shot straight to second place during his opening run, finishing an eventual fifth behind only Jorge Lorenzo (Yamaha), Valentino Rossi (Yamaha), Marc Marquez (Honda) and Cal Crutchlow (Honda).

Stoner was the fastest Ducati rider by 0.147s over Danilo Petrucci (Pramac), in seventh. Factory stars Andrea Iannone and Andrea Dovizioso were eighth (+0.153s) and 13th (+0.605s) whilst debuting the 2016 bike.

Riding the GP15, Stoner was classified 1.490s behind Yamaha pace setter and reigning world champion Jorge Lorenzo, but half-a-second from Valentino Rossi.

Having been off a MotoGP bike for a year, Stoner admitted he was "suffering" physically, "but honestly, I'm not as bad as expected, being off the bike for so long. But also at the same time, we didn't expect to be this competitive, because we're not out there looking for the perfect set up, we're not out there trying to find that last little bit of edge.

"Every step we try to do is huge, trying to get a feeling for each direction and how the bike reacts. We really do have our test schedule and our test plan, which are big steps, but step by step, which got really ruined by the weather today.

"For us to still be consistently doing good lap times, it's positive for us, it's positive for Ducati.

"Quite honestly it was disappointing with the weather," Stoner added. "We did two exits with massively different steps in geometry, to try to get some understanding, and then we had another two steps to do after that, and then rain again, so we had to shorten our program in the afternoon with some dry track time.

"In the afternoon, we were struggling to get enough temperature in the rear tyre, to get some grip, so we had to play around a little bit to get a base setting again that we could compare Michelin front tyres that was really the only thing we did at the end of the day."

Stoner also gave an insight into why there had been numerous accidents at Turn 5.

"There's a little point after probably 45? [lean], that [the front] goes down just a little bit more. It doesn't seem to match with the rear with some of the profiles that we've tested. And that gives everybody a little bit a nervous feeling, and is essentially why people are struggling into Turn 5, a big fast open corner, going in, when the bike goes light. It doesn't like that feeling, and it gets the bike a little nervous, and I think that's when the front wants to break away.

"Everybody has been having a very similar crash there. So we've been cautious of that over the past days. In general, the braking stability isn't quite there, like a Bridgestone, but it has a lot of positives as well. As you get into the corner, you can enter a later, it wants to turn a little more, and then of course you have that fantastic rear grip, so I think there's positives and negatives to everything, but in general, the feeling I've had so far has been good."

The Ducati has more winglets than any other MotoGP bike, aimed at providing downforce at the front of the bike and therefore reducing wheelies. It is an area Stoner will explore further.

"For the future tests we're going to take them off, then put them back on to understand exactly what they are doing," he said. "I can't find any negatives; it doesn't feel heavy to change direction. I think the riders that have come from bikes without [wings] to the Ducati as well, it's quite similar, it doesn't feel any different.

"So at this point, we can only imagine the positives. When I was riding with the small wings years ago in Ducati, we found quite a lot of positives, and some negatives. But I think they've recovered this problem and now it's a little bit more balanced."

Stoner was also asked, if he was still racing, whether this Ducati would have been good enough for him to fight for the title?

"So far, I've been happy with the bike, and this is last year's version," he replied. "I'm looking forward to testing some of the updated parts for this year and seeing what the differences are. Honestly, I'm not racing and I have no plan to, so I can't really comment in that way, but I would say that the bike is definitely championship material.

"I think with a few more steps and little things here and there, to fix some of the weaker points, it can be a very very competitive bike. It does nothing really wrong, it does a things very well, and a few things not quite well enough. So we're trying to improve those things and try and make it a more complete package.

Stoner will not take part in the two remaining official pre-season tests, at Phillip island and Qatar, but will ride in a private test at the Losail circuit just before the season-opening race.