(ANSA) - Rome, August 10 - Italy's nine-time motorcycling world champion Valentino Rossi will leave Ducati next season and return to his former team Yamaha on a two-year contract, the Japanese outfit said in a statement on Friday. Rossi has struggled since joining Ducati at the start of the 2011 season and has not won a race with them yet. The 33-year-old first joined Yamaha in 2004 and won four MotoGP world titles with them, in 2004, 2005, 2008 and 2009. During his seven-year stint at Yamaha he won 46 grands prix. He will be teammates again with Spaniard Jorge Lorenzo, the 2010 MotoGp world champion who is currently top of the riders' standings after winning five of the 10 races so far this season. Ducati said Rossi would leave them at the end of the current campaign on Friday, shortly before Yamaha announced he was joining them.