The 25-year-old Spaniard birdied four holes in a row starting at the 13th, surging while American Tony Finau stumbled from the lead on the back nine, to take command after 54 holes at Muirfield Village in Dublin, Ohio.
Rahm, whose most recent victory came in November at the European Tour’s season finale in Dubai, fired a four-under par 68 to stand on 12-under 204 with American Ryan Palmer and Finau sharing second on 208 after 73s.
Saying he had been hitting the ball too strong during workouts in the tour’s three-month COVID-19 hiatus, Rahm said some swing changes helped set up his success this week.
“The main thing was changing the loft on the clubs,” he said. “Had to get used to it last week. It seems to be paying off now.”
Rahm can overtake Rory McIlroy and claim the world number one ranking for the first time on Sunday with a victory or runner-up finish, depending on how the four-time major winner from Northern Ireland fares.
With a win, Rahm needs McIlroy to finish worse than a two-way share of second. If Rahm is alone in second, he needs McIlroy outside the top 30 and Justin Thomas not to win.
The Jack Nicklaus-hosted event is the sixth tournament since the US PGA Tour resumed following a three-month hiatus due to the COVID-19 pandemic, all events played without spectators.
A major schedule shuffle in the virus-hit season wiped out the British Open, originally set for this week, and pushed the PGA Championship from May to August, where it will be the year’s first major tournament.
Finau and Palmer shared the lead at 9-under, one ahead of Rahm, when the day began.
Rahm sank a five-foot birdie putt at the par-5 seventh but found a bunker and made bogey at the par-3 eighth, then made his charge on the back nine.
Rahm sank a 13-foot birdie putt at 13, dropped his approach just beyond four feet and sank the putt at 14, holed a three-foot putt at the par-5 15th and rolled in a 26-foot birdie putt at the par-3 16th before closing with two pars.
Finau sank a 10-foot birdie putt at the par-3 fourth to grab the lead at 10-under, but his tee shot at the par-5 fifth bounced off the sun-baked fairway and into a water hazard, although he was able to salvage par.
Finau began the back nine with back-to-back birdies, holing an impressive 50-foot putt at the par-4 ninth and reaching the green in two at the par-5 11th to set up a tap-in birdie to reach 12-under and seize a three-stroke advantage.
– Double bogey disaster –
Double-bogey disaster struck Finau at the par-3 12th when he found the rough, needed two chips to reach the green and missed a 10-foot bogey putt.
Finau sent his tee shot into the water again at 14 on the way to a bogey and, after a birdie at 15, took another double bogey at 17, needing two to reach the green from a bunker.
Finau is a six-time PGA runner-up whose only win came at the 2016 Puerto Rico Open.
England’s Danny Willett was fourth on 210 with Australia’s Jason Day and Sweden’s Henrik Norlander sharing fifth on 211.
Reigning Masters champion Tiger Woods, who battled back issues Friday and shot 76 to make the cut on the number, improved to a 71 on Saturday to stand on 218.