* Johnson drops two shots at 17th
* Shares lead with Chappell after 54 holes
* McIlroy two back after a 66 (Adds quotes, details)
Sept 24 (Reuters) - Thoughts of a Dustin Johnson runaway at the Tour Championship were dispelled when he faltered late to fall back into a tie for the third-round lead with fellow American Kevin Chappell in Atlanta on Saturday.
Johnson, the hottest player in the game coming off a victory, squandered a chance to take a stranglehold on the tournament when he double-bogeyed the par-four 17th, compounding a poor drive by clipping a pine tree with his second shot.