The third round was completed Sunday after heavy rain and thunderstorms forced the suspension of play on Saturday.
Bo Van Pelt climbed into third place with a 5-under 67 to move to 11-under- par 205. Tjaart van der Walt is one stroke further back at minus-10.
Purdy, who played all 18 holes of his third round on Sunday, birdied the third and the sixth to get to 10-under dropped a shot with a bogey at the seventh. Purdy recovered quickly with a birdie at the par-5 eighth.
Around the turn, Purdy continued to torch the par-5s as he birdied the par-5 11th, his third birdie on a par-5 during the round. He faltered with a bogey at the 16th, but closed out in style. He birdied the 17th and followed that with another birdie at the par-5 18th, giving him a birdie on all four par-5s.
Johnson, who led entering the third round, struggled throughout. He bogeyed the par-4 second before parring the final seven holes on the front nine. On the back side, Johnson stretched his string of pars to 13 straight before dropping a shot at the par-3 16th.
He did atone for the mistake with a birdie at the par-4 17th. Johnson, however, bogeyed the last to drop back into a share of the lead.
Van Pelt had a shot at the lead but his lone bogey of the round ended those thoughts. He had three birdies and an eagle on the front side to get to minus-11. He could not maintain his momentum, however, as he bogeyed the par-3 12th. Van Pelt took third place alone heading to the final round with a birdie at the 17th.
Danny Ellis, Scott Gump, Bob Heintz, Mark Hensby, Chris Tidland and Vance Veazey are tied for fifth place at 9-under-par 207. Grant Waite and Scott Peterson share 11th one shot back at minus-8.