Valentino Rossi unsure he will be able to match Jorge Lorenzo at Yamaha in 2013

The seven-time champion insists however that the lure of being able to fight at the front again was enough to assuage such concerns.

Such is Lorenzo's form at Yamaha, Rossi said his goals were more inward-looking, his focus being on having fun and re-evaluating his ability rather than race wins and title bids.

"After two difficult and tough years I am not sure I can be as fast at Lorenzo with the Yamaha," Rossi admitted.

"For me I have to be concentrate very much on myself and try to understand how much is my potential in the next years.

"Lorenzo is maybe the best rider, and [has had] a lot of incredible races on the M1.

"I think it will be difficult for me to beat him, but important is to have fun and especially have the chance to fight for the good position, to achieve some important results and try to improve the bike from this level and do some good races from start to finish.

"I know it will be difficult but this is the target."

Rossi also downplayed the potential for he and Lorenzo to revisit the acrimony of their previous relationship as Yamaha team mates.

"I think we will have [fewer] problems than in the past," he said, "because Lorenzo was very fair with my comeback, [he] never gave problems."

Despite his admission that Lorenzo poses a daunting challenge, Rossi insisted the main difficulty he had in leaving Ducati was the failure of a 'great marriage'.

"It is a great pity for my fans and all the Ducati fans because for sure it was a great marriage between the Italian bike and Italian rider," he explained.

"I have to say we are very sad because we give the maximum but we don't achieve the good results. We had some good races, especially this year in the wet at Le Mans, [where Rossi] arrived in the second place and the podium after battling with Stoner.

"You know, for me take the decision was very hard for next year, because in Ducati I know a lot of great people and also Audi arrive and for sure Ducati will change a lot, the situation, in the next years.

"This experience, this adventure already from the start was very difficult and unfortunately we are not able to fix our problem during this year and half and I was never competitive with this bike.

"We are not able to create enough grip from the front tyre and enough contact on the ground, this was the biggest problem from the Valencia test in 2010 and unfortunately after two years we have the same problem.

"But I make my decision because at this point in my career I have to try and have the more competitive bike, the best bike for me, and at the end I did the decision for this reason.

"Tough and difficult moments are very important for to become a better and stronger person. I suffer a lot during this period, we try everything but it was not enough."