Rossi leads MotoGP into the unknown.

MotoGP World Championship leader Valentino Rossi heads for one of only three circuits at which he has failed to win this weekend at Istanbul, as the new 800cc era enters an event shrouded in mystery.

Unlike the opening two rounds, at Losail and Jerez, the MotoGP teams will be racing at a circuit where they have no previous testing experience with this year's 800cc machines. And, not only will they have to set the bikes up from scratch, but new regulations limiting the number of tyres also come thundering into the equation.

That's because the 17 rear tyres and 14 fronts each rider - with the exception of the Dunlop teams - will use this weekend need to be chosen before a wheel has been turned. That wasn't of such significance when pre-season testing data was available, but this weekend Michelin and Bridgestone will have to make an 'educated guess' as to what their riders will need.

"It's the first time this season that we're going to be racing at a track that we haven't tested at," stated Rossi. "With the new [tyre] restrictions it's going to be very important to make the right choices and also we don't know what the weather will do."

For Rossi, who currently holds a nine point lead over Casey Stoner and Dani Pedrosa, the Turkish Grand Prix is the first of two potentially tough races - but victory this weekend would mean that, of the 18 venues on the 2007 calendar, only Laguna Seca in the USA and Misano in Italy (which will this year host a grand prix for the first time since 1993 this season) stand unconquered.

"The next two races, Turkey and China, are ones that I don't like so much and I have never won at Istanbul, but it's a great circuit with some very fast corners," said The Doctor. "If the bike works well then it will be great fun I think. Last year I made an error early in the race and it cost me the chance for a podium, but I won't be making the same mistake this year! We are leading the championship but it's a long season and we have to aim to get on the podium at every race."

But Rossi's biggest headache this weekend, aside from unknowns, could well be a lack of top speed: The Italian's Fiat Yamaha struggled badly against Casey Stoner's Ducati along the main straight in Qatar - and both Istanbul and Shanghai also require potent engine power.

"The difficult thing over the next two races is going to be horsepower, because both Istanbul and Shanghai have very long straights," confirmed Rossi, who won with ease around the twists of Jerez last time out. "We tried some new things at the Jerez test to improve this and I know that Yamaha have been working very hard in Japan so hopefully we will have a little bit of improvement in this area when we get to Turkey."

However, team director Davide Brivio believes that Shanghai, with its enormous back straight, poses the greatest problem - and that Rossi and team-mate Colin Edwards will be on the attack this weekend.

"We head to Turkey looking to attack because China might be difficult for us and we may have to defend more," conceded Brivio. "Of course the big challenge for everybody this weekend is about finding a set-up quickly and making the right choice with the tyres. Istanbul has not been a good track for Yamaha and the cold conditions won't help either, but we have a team of very shrewd engineers and a fantastic relationship with Michelin so we are in a good position to make the difference.

"Even though we haven't tested at Istanbul we have gathered data at a lot of varied tracks during the winter and the bike has worked fairly well everywhere, which is crucially important when it comes to making a consistent challenge for a world championship," he continued. "It is still early in the season but the new M1 seems to have a solid base, so we'll see on the Friday what we need to work on. Both our riders are very motivated after their excellent results in Jerez so we are looking forward to another strong challenge from them this weekend."