Americanus (2) finished third in the 1 1/16 mile dirt stakes.
The race had six starters, following the scratch of South Bend.
Dr Post was riddden by Irad Ortiz Jr. and won in 1:43.07 on the track that is listed as Fast despite periodic rain.
Dr Post was in tight traffic coming out of the final turn. Ortiz found a "split-horses" opening in mid-stretch and Dr Post powered ahead to his second career win in three starts.
He broke his maiden on March 29 at seven furlongs at Gulfstream.
Dr Post, a son of Quality Road, is traained by Todd Pletcher for owner St Elias Stable.
The race was the fourth leg in the Rainbow 6, with a carryover f $145,470.
Winners of the first four legs were:
7th race -- Venezuelan Hug (4) 9-5
8th race -- Scars are Cool (2) 5-1
9th race -- Dr Post (3) 1-1
10th race - New Year (3) 9-2