Hoover knotted the score with 35:16 to play when sophomore Hannah Schoolcraft slipped a shot past Williamstown junior keeper Heather Albertson.
The Yellowjackets again relied on late-half heroics to take the lead when Petty scored with 1:45 remaining with an assist by freshman Ellie Gardner.
While disappointed with the loss and handful of missed opportunities, Hoover Coach Brittany Woods also saw a silver lining in the defeat.
"We have to start capitalizing on all the chances we get," Wood said. "We need to hustle 100 percent while we're on the field.
"We've squeaked by some teams and we've been on a roll for a little bit, but this isn't entirely bad. Sometimes you need knocked down to show that we still need to work and improve what we do.
"We just need to capitalize on our chances and pass the ball around. We need to play our style of game. Sometimes we play a little relaxed and our heads aren't in the game for the whole 80 minutes."
Heather Albertson finished with 11 saves for the Yellowjackets, while Hoover junior keeper Bailey Aab collected 12.
Williamstown is in action again Saturday at 1 p.m. when it travels to John Marshall. Hoover will host Ravenswood at 5:30 p.m. on Tuesday.
Contact Michael Dailey at mike.dai...@dailymail.com. Follow him on Twitter @MichaelDaileyWV.