Two-time major winner Morikawa and world number three Schauffele, seeking his first major crown, were on 15-under 198 after 54 holes at Valhalla with fellow American Sahith Theegala one stroke off the pace.
Morikawa, the 2020 PGA Championship and 2021 British Open winner, had not won since taking the Claret Jug until last October's Zozo Championship. He shared third in April's Masters.
Reigning Olympic champion Schauffele has not won since the 2022 Scottish Open, having had 19 top-10 finishes during his win drought. He matched the lowest round in major history with a nine-under 62 on Thursday.
Englishman Jordan Smith fired a 64 and compatriot Tommy Fleetwood shot 65, an early Sunday sign low scores remained on offer in sunny and warm conditions.
There were 21 rivals within range of the best last-round fightback to win in PGA history, the seven-stroke victory rallies of Justin Thomas in 2022 and John Mahaffey in 1978.
Hometown hero Thomas, a two-time PGA Championship winner, began five back.
World number one Scottie Scheffler, a winner in four of his past five starts including April's Masters, was eight adrift after his first over-par round of the year, a 73 that came a day after his arrest on charges stemming from a traffic incident at the course entrance.
Scheffler faces charges of felony assault of a police officer, criminal mischief, reckless driving and disregarding signals from an officer directing traffic.
Scheffler handled the shock of being taken to jail, booked and released and was back at the course to shoot 66 Friday, then said the charges were a "misunderstanding" and he expected the matter would soon be resolved.
He's set to be arraigned on Tuesday.
Scheffler opened Sunday with a bogey but birdied the fifth and par-3 eighth to make the turn 8-under.
Irishman Shane Lowry fired a 62 to match the major low-round record on Saturday and began two strokes back, trying at 37 to win his second major after the 2019 British Open.
Bryson DeChambeau, the week's driving distance leader and best among 16 LIV Golf players in the field, won the 2020 US Open and was within two along with Norway's Viktor Hovland, last year's FedEx Cup playoff champion who is eyeing his first major triumph.
England's 43-year-old Justin Rose, the 2013 US Open champion, and Scotsman Robert MacIntyre began three adrift, each trying to become the first British winner of the PGA since Englishman Jim Barnes took the first two in 1916 and 1919.
Making a charge was Sweden's Alex Noren, who had five birdies in 13 holes to stand on 11-under overall, where Thomas was after a chip-in birdie at the par-3 third.
Spieth, McIlroy trail
Three-time major winner Jordan Spieth, an American trying to win to complete a career Grand Slam, and four-time major winner Rory McIlroy of Northern Ireland began seven adrift, each needing a record-tying comeback to capture the Wanamaker Trophy.
Spieth hopes to join Tiger Woods, Jack Nicklaus, Gary Player, Ben Hogan and Gene Sarazen as a winner of all four major titles.
World number two McIlroy has not won a major since 10 years ago at Valhalla but has had 20 top-10 major finishes since then.
McIlroy arrived this year after winning his past two starts, a PGA Tour pairs event with Lowry at New Orleans last month and last week's Wells Fargo Championship, where he won for the fourth time at Quail Hollow, host of next year's PGA.
McIlroy opened with a bogey but birdied the par-5 seventh on Sunday.
News broke Tuesday that McIlroy had filed for divorce from wife Erica on Monday after seven years of marriage.
McIlroy made no comments on his personal turmoil other than to say he would be ready to play, and he opened with a 66 but then struggled to rounds of 71 and 68 in rain-softened conditions to fall back.
