Koepka sank four birdies in the final six holes of his front nine after starting his round from the 10th tee where he made bogey.
On his fourth hole of the day at No. 13, Koepka hit a 175-yard approach shot to six feet and then hit another 165-yard approach to just three feet on the next hole. He drained a nine-foot putt on 16 for another birdie.
"I put the ball in the fairway, hit some greens and made putts. You just try to break it down. It is pretty simple and I did a good job of that today," he said.
Two-time major winner Kaymer, who started on No. 10, got hot on his back nine, making two birdies on No. 1 and No. 3 and delivering an eagle on the par-five No. 4.
Kaymer, who won this event in 2010, hasn't been playing golf during the pandemic so he re-watched his 2014 US Open victory for inspiration.
"That helped me to believe that my putting was good enough and that my ball-striking was good enough," he said. "Even though it's a few years back, it's always nice to remember those moments."
Reigning Masters champ Tiger Woods showed why it is never wise to count him out of a major, shooting a solid 68 to join another large group at two under, three shots back of the leaders.
Woods teed off in the morning wave with Rory McIlroy and Justin Thomas as the marquee trio all began their round with birdies on No. 10.
Thomas is competing as the No. 1 player in the world and former number one McIlroy is trying to win a major for the first time since 2014, while Woods is looking for his fifth PGA Championship title.
Woods went to two under through four holes on No. 13 by draining a monster 32-foot birdie putt.
"I felt like it was a very solid round," said Woods. "I made a few adjustments and that is what you have to do."
Bryson DeChambeau also shot a 68 but he had to overcome an equipment malfunction in the middle of his round when he snapped the head off his driver while leaning on it to pick up his tee.
DeChambeau, who is averaging nearly 325 yards a drive after gaining 50 pounds of muscle over the past 10 months, was allowed by officials to fix the club after it was determined that it came off accidentally.
"It was surprising. I just went over to pick up my tee, and leaned on it a little, and the thing just, see ya later," DeChambeau said.
Leading scores after Thursday's first round of the PGA Championship at par-70 TPC Harding Park:
65 - Jason Day (AUS), Brendon Todd (USA)
66 - Zach Johnson (USA), Bud Cauley (USA), Brendan Steele (USA), Brooks Koepka (USA), Scottie Scheffler (USA), Michal Lorenzo-Vera (FRA), Martin Kaymer (GER), Xander Schauffele (USA), Justin Rose (ENG)
67 - Tom Lewis (ENG), Tony Finau (USA), Gary Woodland (USA), Kevin Kisner (USA), Daniel Berger (USA), JT Poston (USA), Li Haotong (CHN), Alexander Norn (SWE)
68 - Viktor Hovland (NOR), Shane Lowry (IRL), Adam Scott (AUS), Adam Hadwin (CAN), Bernd Wiesberger (AUT), Tiger Woods (USA), Brian Harman (USA), Patrick Reed (USA), Lanto Griffin (USA), Bryson DeChambeau (USA), Kurt Kitayama (USA), Keith Mitchell (USA), Paul Casey (ENG)
69 - Kevin Streelman (USA), Nate Lashley (USA), Billy Horschel (USA), Jason Kokrak (USA), Dustin Johnson (USA), Harris English (USA), Joel Dahmen (USA), Collin Morikawa (USA), Matthew Wolff (USA), Matthias Schwab (AUT), Corey Conners (CAN), Abraham Ancer (MEX) Danny Lee (NZL), Shaun Norris (RSA)
70 - Louis Oosthuizen (RSA), Dylan Frittelli (RSA), Henrik Stenson (SWE), Kang Sung-Hoon (KOR), Kim Joo-hyung (KOR), Victor Perez (FRA), Hideki Matsuyama (JPN), Marc Leishman (AUS), Matt Jones (AUS), Emiliano Grillo (ARG), Jon Rahm (ESP), Kevin Na (USA), Tommy Fleetwood (ENG), Rory McIlroy (NIR), Bubba Watson (USA), Jason Dufner (USA), Scott Piercy (USA), Denny McCarthy (USA)
71 - Jim Furyk (USA), Matt Kuchar (USA), Wyndham Clark (USA), Ben Cook (USA), Justin Thomas (USA), Jim Herman (USA), Russell Henley (USA), Talor Gooch (USA), Richy Werenski (USA), Cameron Champ (USA), Chez Reavie (USA), Rob Labritz (USA), Bob Sowards (USA), Lucas Glover (USA), Webb Simpson (USA), Michael Thompson (USA), Matt Wallace (ENG), Cameron Smith (AUS), Joost Luiten (NED), Erik Van Rooyen (RSA), Rory Sabbatini (SVK)
72 - Christiaan Bezuidenhout (RSA), Carlos Ortiz (MEX), An Byeong-Hun (KOR), Ryo Ishikawa (JPN), Zhang Xinjun (CHN), Rafael Cabrera (ESP), Tyrrell Hatton (ENG), Graeme McDowell (NIR), Cheng Tsung Pan (TPE), Phil Mickelson (USA), Steve Stricker (USA), Shaun Micheel (USA), Luke List (USA) Brandt Snedeker (USA), Brian Stuard (USA), Tom Hoge (USA), Chan Kim (USA), Harold Varner III (USA), David Muttitt (USA)
73 - Davis Love III (USA), Jimmy Walker (USA), Rickie Fowler (USA), Cameron Tringale (USA), Troy Merritt (USA), Jordan Spieth (USA), Andrew Putnam (USA), Patrick Cantlay (USA), Keegan Bradley (USA) Alex Beach (USA), Robert MacIntyre (SCO), Ian Poulter (ENG), Sergio Garcia (ESP), Jorge Campillo (ESP), Mackenzie Hughes (CAN), Im Sung-Jae (KOR), Charl Schwartzel (RSA)
74 - Jazz Janewattananond (THA), Matthew Fitzpatrick (ENG), Jeff Roth (USA), Ryan Palmer (USA), Daniel Balin (USA), Marty Jertson (USA), Max Homa (USA), Andrew Landry (USA), Tyler Duncan (USA)
75 - Rod Perry (USA) John O'Leary (USA) Mike Auterson (USA), Danny Willett (ENG), Joaquin Niemann (CHI)
76 - Nick Taylor (CAN), Rich Berberian Jr. (USA)
77 - Jeff Hart (USA) Alex Knoll (USA)
80 - Rich Beem (USA)
82 - Zach J. Johnson (USA)