Thomas fired a five-under 67 to stand on 19-under 197 after 54 holes at Sherwood Country Club, edging ahead of Spain’s Rahm, who fired a 63 to stand on 198, with fellow American Lanto Griffin third on 199 after a 68.
“I can’t exactly go out and try to protect a lead,” Thomas said. “I’ve got to go out and build it. I need to play my game. I can’t do anything about everyone else.”
Colombia’s Sebastian Munoz (66) and Americans Ryan Palmer (66) and Patrick Cantlay (68) shared fourth on 200, with Cantlay bogey-free through three days.
Thomas, the 2017 PGA Championship winner, is chasing his 14th career tour title and his third of the year after the Tournament of Champions in Hawaii in January and the WGC St Jude Invitational in August.
“I’ve been here a couple times and I have been successful,” he said. “I’ll be able to draw on those past experiences tomorrow.”
He opened with back-to-back birdies, followed a birdie at the par-four seventh with a bogey at the par-three eighth. He added birdies at the par-five 11th and 16th holes and curled in a birdie putt from just inside 28 feet at the par-three 17th to seize the lead alone.
“It’s huge,” Thomas said. “A day like today I felt like I was far from my best stuff. To salvage five-under, which when it’s like this you need to do, is huge. I need to go back to what I was doing and get it done tomorrow.”
Standing in his way will be Rahm, who will replace the absent Dustin Johnson as the world No. 1 with a solo second-place finish or better.
The Spaniard, who hit every fairway, went six-under for his first seven holes, birdied to begin the back nine and again at the par-five 13th and par-three 17th in his sizzling 63.
“The course is gettable and lucky for me I started on fire,” he said. “I was able to make a few putts and I was able to keep it going.
“Somebody is going to come out tomorrow and shoot low. They have nothing to lose. I’m going to come out and try to do the same thing tomorrow.”
He seeks his sixth PGA Tour title and third of the year after the Memorial tournament in July and the BMW Championship in August. He is planning on a back-nine title shootout.
“It simplifies things. Somebody is going to come out tomorrow and shoot low,” he said. “They have nothing to lose. I’m going to come out and try to do the same thing.”
World No. 59 Griffin opened with four pars in a row, birdied the 10th and 14th as well, but stumbled with a bogey at the par-three 15th and another at No. 18 to fall two off the pace.
Defending champion Tiger Woods, who won last year in Japan before the event was moved to suburban Los Angeles due to the Covid-19 pandemic, fired a 71 to stand on 213.
Woods, a 15-time Major champion set the defend the Masters title next month, made 10-foot birdie putts at the par-five 11th and par-three 17th to highlight his round.
“It wasn’t very good,” he admitted. “I hit a lot of balls left and made the round a lot harder than it should have been.”
He revealed that the Masters plans to stage its traditional Champions Dinner in an area that allows for more social distancing.
“I think we’re not going to get a lot of the past champions coming because obviously they’re at the at-risk age,” Woods said. “The dinner is, as of right now, planned to go off and still have it.”
Tony Finau, playing his first event after a Covid-19 positive test, was sharing the lead when he made a triple bogey at the par-four 14th to fall back. He shot 69 to stand on 202.