The former NASCAR champion came to Sonoma winless in 20 races since Kansas last fall and has been overshadowed in this season of NASCAR's young new superstars.
The 23-year-old Spieth joined Tiger Woods as the only players in the modern era with 10 victories before the age of 24
The Reds won for just the second time in 15 games.
No. 3 national seed Florida (50-19) will play No. 4 seed LSU (52-18) in the best-of-three finals beginning Monday night.
Daniel Murphy had four RBIs for the Nationals.