NARBONNE, France (Reuters) - Tour de France organisers, adamant that they are winning the war against doping, will be praying for a scandal-free day during Friday's 13th stage.

On Thursday, Italian Riccardo Ricco fell by the roadside after failing a test for the blood-boosting drug EPO. The entire Saunier Duval team withdrew from the race.

Ricco, 24, who is the third rider to test positive for erythropoietin on this year's Tour, had won two stages on the three-week race.

Friday's 182 kms 13th stage from Narbonne to Nimes is flat from start to finish and expected to again favour sprinters.

Briton Mark Cavendish, winner of the three sprints on the Tour this season, will be closely watched by rivals still without a win, such as Spain's Oscar Freire, German Erik Zabel and Belgian Gert Steegmans.

The last sprinter to win four stages on the same Tour de France was Italy's Alessandro Petacchi in 2003. Petacchi is not taking part in this year's Tour because of a doping suspension.

(Reporting by Julian Pretot; Editing by John Mehaffey)