Rossi, who has won nine of the eleven Grands Prix this season, needs to finish 11 points in front of Ukawa to clinch the title with still four rounds remaining. If Rossi wins the race and Ukawa does not join him on the rostrum in second or third place the Italian will be the first winner of the MotoGP crown since the class changed its regulations to allow 990cc prototypes to enter at the beginning of this season. It would be Rossi's fourth world title having already won the 125, 250 and 500cc crowns, but it will not be that easy around the circuit where he won the last 500cc Grand Prix of 2001 by just 0.143 seconds from the Yamaha of Carlos Checa, in a race that was run in two parts because of the rain.

Checa will be a big threat on Saturday, finishing second to Rossi after starting from pole position riding the YZR-M1 Yamaha in the Estoril rain last week. His Yamaha team-mate, for just five more races, Max Biaggi will also be desperate to spoil his great rival Rossi's party. Biaggi gave Yamaha their only victory of the season at Brno in the Czech Republic and only trails second placed Ukawa by 12 points in the championship.

Ukawa has finished third in the last three Grands Prix and will be looking for at least a repeat in Brazil where he started the race in pole position last year, to prevent his team-mate clinching the title at such an early stage. Daijiro Kato won the 250cc race at Rio for the last two years and returns with the RCV Honda Gresini. He made a brilliant debut on the machine in Brno where he finished second but showed his dislike for the rain when he crashed out of 12th place in Estoril. The Japanese rider will be praying for sunshine on Saturday.

Estoril also provides unhappy memories for Spaniard Sete Gibernau, who crashed while leading with just four and a half laps remaining. The Suzuki rider damaged his shoulder but should be fit to ride the GSV-R machine that improves every race. American Kenny Roberts clinched the 500cc title for Suzuki at Rio two years ago and returns full of confidence after his best result of the season at Estoril, where he finished fourth.

The long straight that incorporates the CART racing oval in Rio will not make life easy for the 500cc bikes, with the bigger 990cc machines having the edge in a straight line. However, Brazilian veteran Alex Barros will be hoping to give the home crowd something to cheer about, riding the NSR500 Honda Pons after finishing second and fourth in the last two years. He's currently fifth in the championship and will want to impress with the chances of a ride on the RCV Honda in the last four races.

Both Yamaha Tech 3 riders Olivier Jacque and Shinya Nakano could be having their last but one rides on the YZR 500cc Yamahas with M1 four-strokes promised to them for the last three Grands Prix. Norick Abe continues to pile on the points riding the Yamaha D'Antin, but the Japanese rider is fighting for his future and will be looking to consolidate or improve his sixth place in the championship. Nineteen year-old American John Hopkins continues to impress on his debut season on the Yamaha but his Red Bull WCM team-mate Australian Garry McCoy is struggling after returning from injury. It's an important final part of the season for him.

Both Jeremy McWilliams and Nobuatsu Aoki have put in some tremendous performances on the three-cylinder 500cc Proton KR that really do not reflect their championship positions. It will be tough for them in Rio however, around a circuit that will almost certainly favour the larger machines.

Spaniard Fonsi Nieto may have grabbed a brave 250cc victory in the Estoril rain but Italian Marco Melandri's second place was enough to keep him on track for his first world title. Riding the factory Aprilia, he leads Nieto by 32 points going into the 27-lap race at a circuit where he has a good record. He's finished second and third in the last two 250cc races and second in the 125 race three years go. It was Nieto's third victory of the season riding the Aprilia and he will have to keep on winning to pressure Melandri if he has any chance of winning the title.

Italian Roberto Rolfo finished a superb fourth in Estoril despite suffering a ten second penalty for a start line infringement, to hold onto a comfortable third place, 30 points ahead of the in-form Argentinean Sebastian Porto. Riding the Yamaha, he's finished on the rostrum in the last three races and started from pole for the first time in Portugal.

Frenchman Arnaud Vincent's fourth victory of the season in Estoril has made him favourite for the World 125cc Championship. While the Aprilia rider celebrated in the rain it was doom and gloom in the Gilera camp of world champion Manuel Poggiali. He went into the race leading the title chase but crashed out of fourth place leaving Vincent with an impressive 23-point advantage going into the race where Poggiali clinched his world title last year. Another faller in Estoril was 16-year-old Spaniard Daniel Pedrosa but he remounted the Junior team Honda to score six vital points. He kept his championship chances alive and trails Poggiali in second place by just five points.