Rainey finished in 46min 12.307sec, 0.008sec ahead of Schwantz. Beattie was third and Itoh fourth.
'I got a bad start and rode like an old woman to the first corner,' Rainey, who worked his way up from eighth in the first lap, said. 'But I was on the best bike. The tyres were sliding, but they were predictable, I could ride with real aggression.'
Rainey and Beattie pulled away in the penultimate lap, but Schwantz fought back. He passed Beattie on the last lap, but could do no more than chase Rainey across the line.
The former world champion, Freddie Spencer, crashed in qualifying on Friday, breaking bones in his wrists and injuring a finger that required plastic surgery. He will be out for at least a month, but may be back for the German Grand Prix in June.Reuse content