Conners started a grey, rainy day at Oak Hill sharing top spot with world number two Scottie Scheffler (26) and Norway's Viktor Hovland (25) but with nine to play finds himself alone two clear of the pack at six-under.
While Conners turned in a disciplined error free first nine Scheffler was self-destructing in the tough conditions piling up four bogeys to tumble down the leaderboard suddenly five off the pace.
Bidding to become the first Canadian man to win a major since Mike Weir two decades ago at the Masters, Conners delivered a clinical controlled display starting his day with seven consecutive pars before taking his first and only birdie at the eighth.
Lurking two back are Hovland and Briton Justin Rose (42).
Like Scheffler, Hovland looked headed in the wrong direction early with back-to-back bogeys at four and five but steadied the ship with a birdie at eight to get to four-under.
Starting the day four off the lead Rose put himself in a deeper hole with a bogey at the second but he would make no more mistakes picking up four birdies through 13 holes to charge into contention.
From the first players off at 8:10am ET (13:10 CET) until the last pairing of Scheffler and Conners almost seven hours later the East Course had been pounded by often torrential rain that dumped more than half-inch (12.7 millimetres) on the already challenging layout.
Scheffler, Conners and Hovland had shared the overnight two shot lead and held the same advantage as they teed off, the rain effectively putting the breaks on any serious challenges.
The forecast was calling for rain to subside during the leaders round but they will still have to contend with a water-logged layout despite an army of course workers pushing water off the greens with squeegees in a heroic effort to keep the course playable.
The downpour did not deter spectators but the large crowd appeared to be in an ugly mood by the time Brooks Koepka (33) and Bryson DeChambeau (29) teed off in the third last grouping, loudly jeering the LIV Golf standard bearer.
Once bitter rivals, the pair are now LIV Golf teammates united in the Saudi-bankrolled venture's feud with the PGA Tour, which they left to sign on with the big money rebel circuit.
The boos certainly did not impact the two major winners, Koepka going one-under through 11 to sit just three off the lead and well within striking distance of a third PGA Championship.
One shot further adrift four back sits DeChambeau also with seven to play.
Starting on even par Rory McIlroy looked poised to make a charge with birdies at three and five but the Northern Irishman gave it all back and more with three bogeys over a four hole stretch heading into the turn.
The world number three, as he has all week, continued to grind away carding birdies at 12 and 13 to quickly get back in the hunt at one-under.
The late pairs certainly knew what was ahead as they watched drenched early starters come home.
Defending champion Justin Thomas (30) began the day at five-over and doubled that to 10-over with a five-over 75 that included six bogeys and single birdie.
Six-time major winner Phil Mickelson (52) also had a 75 joining Thomas near the bottom of the leaderboard.
Jordan Spieth (29), needing a PGA Championship to complete the career Grand Slam of all four majors will likely have to wait another year to get it done as a respectable one-over 71 still left him well off the pace on six-over.
"It's just really difficult to keep everything dry, and so it starts with a grip, and then from there, if you get water on the ball or the driver face, the ball can really go anywhere," said Spieth.
"There was two drives where right when I hit them, I looked up and I was like, oh, man, that's a water shooter."
World number one Jon Rahm's (28) miserable PGA Championship continued as the Spaniard battled to a two-over 72.
The Masters champion opened with back-to-back bogeys and stumbled through the turn at five-over before rallying with three birdies but remained well back at six-over going into Sunday.
