The superstar trio began off the 10th tee at Bellerive Country Club with thousands of spectators lining the fairway, standing 10 deep in spots just to glimpse the master shotmakers.
Instead, Woods endured an early horror show, McIlroy was grinding and only Thomas was showing major champion form.
Second-ranked Thomas, coming off a WGC victory Sunday, sank birdie putts from five feet at 11 and seven feet at 12, rescued par from a bunker at the par-3 13th and added another birdie at 15 to share the lead after his first nine holes.
Fellow Americans Rickie Fowler and Stewart Cink and Spain’s Rafa Cabrera Bello were also at three-under par.
Thomas, 25, could become the first player since McIlroy in 2014 to make the PGA his second win in back-to-back weeks and join Woods as the only men since 1937 to win consecutive PGAs.
Woods, a 14-time major champion in a comeback season after spinal fusion surgery, struggled from the start, finding right rough off the 10th tee and making a nervy five-foot bogey putt.
The 42-year-old former world number one found water with his second shot on the way to a double-bogey six at 11 but dropped his approach to two feet and birdied the 12th. He was undone again at the par-3 16th, needing two chips to escape the rough and tapping in for bogey, but birdied 18 to stand two-over for nine holes.
Woods briefly led in the final round of last month’s British Open, but he hasn’t won a major since the 2008 US Open or any title since 2013.
Four-time major champion McIlroy, whose last major triumph came in 2014, found a bunker at 10 and made bogey, then sank birdie putts from eight feet at 11 and 12 feet at the par-3 13th only to close his first nine with a bogey to stand on level par.
Cink, the 2009 British Open champion, started on the back nine and birdied three of his first four holes in search of his first victory since defeating Tom Watson in a playoff at Turnberry for his first major title.
Fowler and Cabrera Bello seek their first major titles.
– Lyle death touches many –
Many players teed off with a heavy heart after Wednesday’s death of Australian Jarrod Lyle, a former tour player who lost his battle with leukemia at age 36.
“The courage shown through his battle is an inspiration to all of us. He made the world a better place. RIP mate,” McIlroy tweeted.
Lyle, whose best US PGA Tour finish was a share of fourth in 2012 at Riviera, played 121 PGA events from 2006-2016. He was diagnosed with leukemia as a youth and twice thought he had beaten cancer only to have it return last year.
“Incredibly sad news about Jarrod Lyle,” tweeted British Open champion Francesco Molinari. “Tough times for everyone on tour losing someone so special.”
Italy’s Molinari was set for an afternoon start over the 7,316-yard, par-70 layout alongside this year’s other two major winners — Americans Patrick Reed from the Masters and Brooks Koepka from the US Open.
“You were and always will be a fighter and will be missed by so many people. Rest In Peace Jarrod,” Reed tweeted.
Australian Jason Day, the 2015 PGA Championship winner, started on the back nine alongside five-time major winner Phil Mickelson and 2011 PGA winner Keegan Bradley.
Day and Bradley were each one-under through nine while 48-year-old left-hander Mickelson, trying to qualify on points for a 12th consecutive US Ryder Cup squad, was two-over through nine holes, needing four shots to reach the green on the way to a double bogey at 15 and finding a greenside bunker to bogey the par-4 18th.
© Agence France-Presse