That left Woods five strokes off the pace of U.S. compatriot Koepka, who birdied five of his first nine holes to make the turn on 13-under par, three shots ahead of Americans Rickie Fowler and Gary Woodland with Australian Adam Scott, the 2013 Masters champion, and American Kevin Kisner another stroke adrift.
The quiet American looked set to match the largest 54-hole lead in tournament history when he bolted five shots clear after 13 holes from a large group including Tiger Woods at Bellerive.
Before the rain hit, incredibly low scores were being put up.
Woodland enjoyed a one-shot lead over compatriot Kevin Kisner, who bogeyed the last to shoot 64, with Schwartzel three off the lead alongside World No.1 Dustin Johnson and Thomas Pieters after eight birdies and a bogey in his 63.
Finau and Hocknull will resume their rounds at 6 a.m. MT, weather permitting.
Matt Wallace is playing in his first PGA Championship and the 28 year old Londoner marked the occasion with a truly memorable shot to the par three 16th, as his five iron hopped twice and rolled into the hole for an ace.
"I didn't think we were going to get called off yesterday and I was going along lovely so I was probably a little bit disappointed".
Even those failing to make Friday's 36-hole cut will walk away with $3,100.
How to watch the US PGA Championship on TV, online and overseas
With points counting double and a big purse, the victor is virtually assured a spot on the team. This weekend at Bellerive is the last chance to earn points and automatically make the team.
Manafort ex-aide Gates testifies at trial on Cyprus accounts
Under cross examination, she said at the time she believed Manafort was directing Gates' actions and "knew what was going on". Manafort's defense attorneys have sought to paint Gates as an embezzler, a liar and the instigator of any criminal conduct.
Baker Mayfield throws bullet for another TD
Spencer Drago was called for holding, though, setting up third-and-18. Mayfield closed 11-of-20 for 212 yards with two scores on the night.
The four-time US PGA champion is 10 under on the front nine for the tournament, but two over on the back nine this week.
"I knew the golf course would be very playable this morning so I thought if I came out and shot two under it would be a very good score", Lowry told RTE Sport. "Most majors the weekends get hard, but I think this course you're going to have to keep shooting birdies". "The key is to get the ball in the fairway and attack from there".
Woodland, who started working with Molinari's putting coach Phil Kenyon at Carnoustie, had held a one-shot overnight lead and recovered from a bogey on the 14th with a brilliant eagle from five feet on the 17th.
"I'm happy with the way I scored considering how I played", said Rose, who did not play a practice round after recent back spasms.
"I feel like I played well today".
A superb approach to the green on the par-four 12th left him with a two-metre putt for birdie, which he duly nailed.
"It kind of reminded me of being in the vortex a lot years ago", said Stewart Cink, who was grouped with Woods on Saturday. If you're hitting it down in the fairway with the way the greens are designed, you can get the ball to about 15 feet on nearly every hole.
While showing a replay of Woodland draining a long putt, Lerner noted: "Mizzou fans have seen this before - a Kansas Jayhawk making everything in Missouri".