Dewease, a three-time winner of the Open, finished third.
"We gave it everything we had," he said. "With it being wet early we didn't know how far to go with it (car) and we didn't go far enough. But we left everything we had on the track."
Paul McMahan was fourth and Craig Dollansky fifth.
Sixth through 10th were Buckwalter, Greg Hodnett, David Gravel, Joey Saldana and Donny Schatz.
Pittman holds a 57-point lead over Schatz with four races left in the season to determine the 2013 WoO champion.
Heats for the 62 410 sprint car were won by Rahmer, Sammy Swindell, Gravel, Rodney Westhafer and Logan Schuchart. Kerry Madsen won the D-Main; Justin Henderson won the C-Main and Hodnett won the B-Main.
Buckwalter won the Dash while fast time of the night went to Saldana with a lap of 17.192 seconds.