Reigning six-time MotoGP world champion Valentino Rossi expects 2008 runner-up Casey Stoner to be his number one rival during an "even more difficult" 2009 season.

Stoner joined only Kenny Roberts Jr (2000) and Nicky Hayden (2006) in beating Rossi to a premier-class title when he dominated the 2007 world championship, during a stunning first season with Ducati.

2008 proved more problematic for the young Australian, who suffered early season set-up problems before twice falling from the race lead, at Brno and Misano. By contrast, Rossi was the epitome of consistency, finishing off the podium just twice during 18 races and winning nine events to wrap up his first title since 2005.

Nevertheless, Stoner proved his raw speed remained intact by taking more pole positions and fastest race laps than any other rider on his way to second in the world championship - and Rossi is backing him to come out fighting in 2009.

"I think that the number one rival will be Stoner," said Rossi, during an interview marking Fiat Yamaha's online team launch. "Then also Pedrosa will be very fast, and of course [Rossi's team-mate] Lorenzo."

Despite finishing 93 points clear of Stoner in the final standings, Rossi has claimed that the 2008 title was his hardest ever - but he now expects 2009 to be even tougher.

"2008 was difficult because it was the year in which I had to give the most of myself in order to win," he explained. "In 2009 it will be even more difficult because my adversaries, who suffered in 2008, are now out for payback and will be looking to beat me. I think it's going to be even harder than 2008!"

Rossi's team manager Davide Brivio agreed with his rider, both in terms of their number one rival and the size of the challenge they face in keeping their crown.

"Stoner, who declared he'll be more aggressive," said Brivio, when asked to name his biggest threat.

"Certainly next year will be more difficult, partly because in 2008, after two hard years, we were not among the favourites," Brivio stated. "Maybe some people thought that it was the beginning of Valentino's decline from a competitive point of view. Instead we managed to have a great, redeeming season, so now everybody is waiting for us, especially our rivals. This is normal; when you don't win, you become more hard-bitten.

"This year Stoner will be more motivated and aggressive, as will his team. Pedrosa will try to do well and also Lorenzo, after his debut, will want to profit from his experience. It'll be hard, everybody is lying in wait for us. But this challenge is the good thing about sport, so we accept it and look forward to seeing whether Valentino will manage to hold these kids off!" he added.

Meanwhile, for the first time since joining the premier-class, Rossi could find himself facing a serious world championship challenge from the other side of his pit garage - should Jorge Lorenzo, who took one win and six podiums on his way to winning the 2008 rookie of the year title, eradicate the accidents that cost him so dearly during his debut season.

"I do think that Lorenzo will be one of my strongest rivals, because he has a year's more experience and this year there is also the monotyre, so we will start equal. He will be a very hard rival to fight," Rossi said.

Rossi, although obliged to mention his team-mate in a list of potential challengers, is clearly taking the threat posed by Lorenzo seriously - and the wall between the two Fiat riders, needed due to different tyre brands in 2008, will remain in place.

All the indications suggest that Rossi was in favour of the wall staying - a sure sign that he views Lorenzo as a serious rival.

"In our team there are two very strong riders and both of these riders are setting out to win the world championship," said Rossi of the wall. "We found the arrangement to work very well in 2008 and so we will continue with it also in 2009."

"When two riders fighting for victory are in the same team, there can be some tension between them," added Brivio. "This solution [the wall] lets us manage the situation in the best way, because as they say we have 'two roosters in the hen house'. Maybe it is better to keep them separated and let them be at their best during the races."

Brivio also pointed out that many teams have an 'invisible wall' dividing their pit garages, and expects other teams to follow Yamaha's example of two team managers and a wall divding the pits.

"I think others will follow our example," he said. "Even if they don't have a real wall, sometimes there is an invisible wall and that can be even thicker than ours, which is visible. Other teams are experiencing tensions between their two riders, physically there is no wall between them but in reality it is almost as if there is one.

"I think the atmosphere in the garage will remain the same as in 2008; there was a separation and this helped, because Lorenzo will be one of the toughest adversaries in the challenge for the title. He showed this last year although he committed a few errors due to slight inexperience besides some unlucky falls. He's certainly thinking about his 2008 mistakes this winter and he'll try not to repeat them; he will be one of the toughest rivals, but we already knew this. He is very motivated.

"For him and for the other riders beating Valentino Rossi is an achievement, so we'll have to put up a defence, or even attack these young riders who are hoping for the recognition they would receive if they managed to beat Valentino Rossi!" admitted Brivio.