Woodland edged ahead at Bellervire late on Thursday, and the American continued his momentum by registering four birdies and an eagle on Friday.
However, it was Brooks Koepka and Charl Schwartzel who produced the rounds of the day, with the pair hitting scores of 63 to move into third and tied fourth respectively.
Kevin Kisner is in second place after recording a 64, while world number one Dustin Johnson has moved himself into fourth spot after shooting a 66.
Many of the late starters in St Louis are still to complete their second round due to thunderstorms, with Tiger Woods in a share of 23rd place after beginning his round with three birdies from seven holes.
Rory McIlroy is on level-par after the same number of holes - leaving the Northern Irishman facing a battle to make the cut - but Tommy Fleetwood is safely into the weekend after finishing at one-under-par.
US PGA CHAMPIONSHIP LEADERBOARD
1. Gary Woodland (-10) 2. Kevin Kisner (-9) 3. Brooks Koepka (-8) 4. Dustin Johnson (-7) = Charl Schwartzel (-7) =Thomas Pieters (-7) =Rickie Fowler (-7) (after 10 holes)