UPDATE: After speaking to the media, Casey Stoner and Ducati held their engineering debrief where it was decided that he will not test on Sunday, as originally planned, but instead return to the track during one day of next week's official test. Tuesday is thought most likely.

It means Stoner will be riding alongside the 2016 MotoGP race riders, with official timing...

Casey Stoner made a 'very successful' return to Ducati during the start of a private MotoGP test at Sepang on Saturday.

Riding a Desmosedici for the first time since 2010, Ducati's only MotoGP champion left the pits at 11:15am for the first of a series of short runs, all using last year's GP15 machine fitted with 2016 Michelin tyres and the standard ECU.

After finishing at just after 6pm, Stoner answered questions from a small group of media in the Ducati hospitality.

"The whole build-up to this test... it definitely means something. Meeting everybody, the new people in the department and meeting up with old friends again. It's been a lot of nice emotions," said Stoner. "It's been a very successful day today to be honest. First time working with everybody and seeing how they go about things. There was the utmost professionalism and everybody has been working really well.

"Those first laps - I didn't expect to be so comfortable with the bike so quickly. You can definitely feel that it's got some grunt. It's powerful. You can feel the good points and some points that need to be worked on, like any machine.

"In my opinion it's been a very successful first day, especially moving forward to the point where I can really give them feedback already by the end of the first day. Normally that's quite difficult to do until you are really comfortable."

The 30-year-old Australian said he was most impressed by the power of the Desmosedici, especially after only riding a Superbike for the past twelve months: "Just grunt everywhere" - but stressed that it was well controlled. He also liked the aerodynamics, with no wind resistance when tucked in, plus the stability at the braking point and in slow corners.

The bike did however feel a little "alien" at the beginning, in terms of getting feedback from the front. "We didn't know if it was tyres or chassis, then we just did a couple of changes with suspension and I immediately got more feeling with the bike and we moved forward from there."

The front feeling is not yet perfect, but Stoner pointed out "I haven't ridden one of these bikes in over a year. Haven't ridden Michelins. New bike. Everything. So I'm really surprised by the end of the day how we got on with it.

"There is room for improvement, but that excites me. When there's something that we know feels like a pretty good package and still has more room for improvement, that's a good sign. Every step we made today was in the right direction and we were able to say definitely how each change made things better or worse in each area."

Stoner admitted the bike felt "hugely" different from his past Ducatis, but "I'm sure any of the manufacturers if you go back six years, it's a massive difference."

Stoner spent four years at Ducati before switching to Honda, where he won a second title in 2011. After retiring at the end of 2012, Stoner conducted some test riding duties for HRC, but has now been tempted back to Ducati as a brand ambassador and test rider.

By far Ducati's most successful grand prix rider, Stoner won the MotoGP crown in his first (2007) season with the team and took 23 wins by the time he left for Honda - where he racked up a further 15 race victories.

But why did Stoner decide to return to Ducati?

"We've had our differences in the past, with people who are no longer there especially," Stoner replied. "Things weren't going the way we would have liked. I think some of the people who were there maybe got a little bit complacent with what I was doing. We weren't seeing any sort of budgeting for [bike] improvements etc in the past.

"The structure and everything at Ducati has completely changed. I've always had a big soft spot for Ducati, especially the MotoGP team. I've got a fantastic relationship with most of the guys and already know the majority of the mechanics.

"This is something that I feel I can make a difference. I'm not here to kick-start my career and really push through again, but what I'd like to do is make a difference for the riders that are there. Get them a little bit more comfortable and hopefully further towards the front, pushing for better results.

"If I can do that I'm going to be very, very happy. But of course my data and my information isn't necessarily going to translate into results, especially when my feelings are a little different maybe to the other riders.

"I'll do whatever I can for Ducati. I'm going to be putting a lot of effort in."

Ducati has not won a grand prix since Stoner's departure at the end of 2010:

"I would like to try and change that stat. I've always felt very sorry for Ducati in that way because the mentality changed from Ducati being extremely competitive and all of sudden people think it is not quite on that edge. For me that is just a mental thing.

"Everybody has been putting the work in, last year they started very strong but we also know that [Qatar] track can be quite good for the Ducati - in the past anyway with me. So we'll see how things go.

"Until we try the new bike and see where it is at the start of the season we don't know what to expect yet. But some of last year's results were impressive. With Andrea Iannone getting some more experience, I think Dovi could have had some better results, he got a little unlucky, crashing with the front a few times. If he starts strong, gets confident there is every chance they could be pushing very good, with the bike they have now."

Ducati's race department was heavily restructured in the aftermath of Valentino Rossi's miserable tenure and Stoner's return was overseen by Ducati Corse general manager Gigi Dall'Igna, who took over the design department in 2014.

"Gigi is straight down the line. As a lot of people know that's me, straight down the line. Some people don't like it, but that's me. It's been fantastic [working with Gigi] so far," Stoner said.

Stoner was riding alongside Ducati development rider Michele Pirro, who focussed on the new (and unpainted) GP16, to be used by race riders Andrea Dovizioso and Andrea Iannone in next week's test.

It is not yet clear if Stoner will test the GP16 and there have been conflicting views on whether he will ride at some point during next week's official test.

Ducati said Stoner set a best lap time of 2m 2.1s on Saturday. The official Sepang race lap record is a 2m 0.6s, by Yamaha's Jorge Lorenzo in October's grand prix when the best Ducati lap was a 2m 1.493s by Dovizioso, using Bridgestone tyres.

Track conditions were not ideal today with only a handful of riders to clean the surface, and Stoner was far from the limit in places.

"Lap times for me were surprising. A lot better than I expected. At some points I was quite slow and cautious and the lap times came very easily so I was happy about that," Stoner said.

"I've only ridden a MotoGP bike three or four times in the last three years. That's not a lot. And just getting used to reading and understanding what's happening and relaying it to the engineers takes a bit of time. We might end up on the GP16 [tomorrow], but there are no plans as yet."

The bigger question remains whether Stoner, whose offer to replace the injured Dani Pedrosa at Repsol Honda last season was surprisingly turned down, could be tempted to do a wild-card race for Ducati.

Stoner skilfully dodged a question about which MotoGP track he would most like to race at again: "Laguna Seca!"

Stoner and Pirro will test again on Sunday, with the official MotoGP test (including lap times) then starting on Monday. Japanese Honda and Yamaha test riders were also on track today.