Lorenzo, who started from the front row, passed fellow Yamaha rider Rossi on the ninth of 24 laps on the 4.8-kilometer Motegi circuit and held on to win his first race of the season with a time of 43 minutes, 47.238 seconds. [break]
Rossi, who won the 2008 Japanese GP, finished second, 1.304 seconds back.
Spain´s Dani Pedrosa was third, 3.763 seconds off the pace on a Honda while Ducati rider Casey Stoner of Australia, winner of the season-opening Qatar GP last week, was fourth.
Saturday´s qualifying was canceled due to rain and the starting grid was determined by Friday´s free practice session. The track was dry for Sunday´s race.
Japanese rider Yuki Takahashi crashed out of the race in a collision with American Nicky Hayden on the second lap.
In the 250cc race, Spain´s Alvaro Bautista passed Japan´s Hiroshi Aoyama with six laps to go to win his first race of the year.
Bautista finished the 23-lap race with a 5.889-second lead over Aoyama, who won the 250cc race at Motogi in 2005 and 2006. Mattia Pasini of Italy finished third with a time of 42:28.320.
Pole sitter and reigning 250cc world champion Marco Simoncelli punctured a tire early in the race and finished 17th.
Italy´s Andrea Iannone, starting from pole position, won the 125cc race after crossing the finish line with a 1.346-second lead over fellow Aprilia rider Julian Simon of Spain. It was Iannone´s second win of the season. Spain´s Pol Espargaro was third on Derbi.
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