Preview - Rio Grand Prix.

World Champion Valentino Rossi arrives in Rio not only leading the World MotoGP Championship, but also with an exemplary record at the Autodromo Nelson Piquet: The Italian has won the last four premier-class races at the 3.065 mile circuit on the outskirts of Rio and has never been beaten.

The Rio Grand Prix has decided several world championships in recent seasons and is one of the most eagerly anticipated on the calendar, but this weekend it takes on extra significance as Rossi and Sete Gibernau continue an intense struggle for the series lead, which sees the two riders level on points - but Rossi on top of the standings by virtue of a superior win ratio.

The Italian's latest victory at Assen last Saturday was his fourth of the season and his third in succession, making him the first Yamaha rider to score a hat-trick of consecutive wins since Eddie Lawson in 1986.

With six wins from six starts at Rio in the 125cc, 250cc and MotoGP classes, there is every reason to believe the World Champion, who won the second of his three premier-class titles here in 2002, can extend his run to four - especially considering a relatively strong record for Yamaha at this circuit, with past wins for Luca Cadalora in 1995 and Norick Abe in 1999.

Rossi's form marks a remarkable turnaround in fortunes for the Japanese factory, who are now leading the constructors' championship, and his confidence appears to be breeding. factory colleague Marco Melandri finished on the podium at Assen for the second successive Grand Prix, the first time Yamaha have had two riders on the podium at back-to-back races since the start of 2000. Melandri has now scored nine more points in the first six races of this year than he did in the whole of 2003.

Back at Honda, Gibernau, who finished second to Rossi at Rio last year, was bitterly disappointed to lose out at Assen - and knows HRC are going to have to come up with something special to prevent Rossi and Yamaha winning the title.

To illustrate that point, Max Biaggi is a rather lonely third in the championship on his Camel Honda, sandwiched between Rossi and Gibernau and the chasing pack headed by Carlos Checa who has a bevy of riders breathing down his neck. Melandri is just two points adrift of Checa and shares sixth place with former World Superbike Champion Colin Edwards who is getting to grips with the Telefonica Movistar Honda with top six finishes at the last two races.

Brazilian Alex Barros returns home after two disastrous races on the Repsol Honda. The veteran has crashed out of the last two grands prix while chasing podium finishes and he'll be seeking a change of fortune in front of his home crowd. His team-mate American Nicky Hayden had crashed out of the two previous races from front row starts but got his season back on track with a hard earned fifth, behind Biaggi in Assen.

It's still tough going for the Marlboro Ducati duo of Loris Capirossi and Troy Bayliss. Former 125 and 250cc Champion Capirossi has finished every race this year but has been nowhere near the podium as he's struggled to adjust to the 2004 machine. Bayliss has failed to finish the last two races and is 12th behind the impressive Spaniard Ruben Xaus, who's had a great season so far on the 2003 D'Antin Ducati.

Meanwhile, three Japanese riders could spring a surprise on Sunday. Norick Abe won the 500cc race for Yamaha in 1999 while Makota Tamada finished third on his Rio debut last year riding the Camel Honda to secure his first podium finish. Last week Shinya Nakano gave Kawasaki an enormous boost by qualifying in fifth place on the Bridgestone - shod machine at Assen.

Just four years ago Kenny Roberts clinched the world 500cc title for Suzuki in Rio and how both the American and his team would like to return to the glory days on Sunday after another tough season for him and team-mate John Hopkins.

The 250cc World Championship could not be closer with just one point separating Frenchman Randy De Puniet and Spaniard Dani Pedrosa after Assen.

De Puniet was a disappointing fourth, riding the Safilo Carrera - LCR Aprilia while Pedrosa rode through the field to finish second after a bad start on the Telefonica Movistar Honda. However, the rider who must be favourite to win the 22 lap race on Sunday is Argentinean Sebastian Porto riding the Repsol - Aspar Aprilia. He won at Assen to consolidate his third place in the Championship, 23 points adrift of Pedrosa and also was victorious at Rio two years ago.

The last three 125cc grands prix have been amazing and the 21 lap encounter on Sunday promises more of the same as Italian teenager Andrea Dovizioso defends a 16 point lead. Spanish teenager Jorge Lorenzo won in Assen after a frantic last lap and he also brought Derbi victory in Rio last year.

Former World Champion Roberto Locatelli, the 'old man' among the youngsters, is second in the Championship with two more teenagers Casey Stoner and Hector Barbara sharing third place.

Shifted from the traditional Saturday to Sunday this season, the Rio Grand Prix gets underway with the first free practice for the 125cc class at 9am on Friday morning.

However, there is a slight change to the schedule on race day, with MotoGP due to take place at 11:30 am, followed by the 125cc class at 1pm and the 250cc race at 2:15 pm local time.