After two chilly days bit hard at Aronimink Golf Club, Saturday gave way to bright sunshine and Rahm and Rory McIlroy were among the heavyweights to take advantage.
Chasing the third leg of the career grand slam, having already won the Masters and US Open, Rahm fired a three-under-par 67 to surge to four under for the championship.
He had a golden chance to claim the lead outright only to register a rueful three-putt bogey at the last hole.
Rahm, though, was still safely in the clubhouse atop the leaderboard alongside unfancied German Matti Schmid and Canadian Nick Taylor, who both matched the low round of the week with sizzling five-under 65s.
McIlroy is one shot back at three under after continuing his stirring fightback at Aronimink Golf Club outside of Philadelphia.
A four-over 74 start had left the back-to-back Masters champion in a tie for 105th after the opening round.
But McIlroy declared himself "right back in it" after climbing to a share of 30th at one over on Friday.
Then, after starting his third round five shots back of Americans Maverick McNealy and Alex Smalley, McIlroy reeled in the halfway co-leaders with six birdies in his first 13 holes.
Bidding to become the first player since Jordan Spieth in 2015 to win the season's first two majors, the Northern Irishman had to drain a clutch 12-foot putt to keep the lead on the beastly 551-yard 15th hole, the longest par-4 in major championship history.
McIlroy then dropped a shot with a sloppy bogey on the par-3 17th but still led when he signed his birdie-laden scorecard as the late starters had to contend with growing wind gusts.
Two-time major winner and 2024 PGA champ Xander Schauffele is also at three under after matching McIlroy's 66.
Englishman Aaron Rai was the on-course frontrunner at five under late in his round as some 14 players enjoyed a slice of the lead at various stages on Saturday.
Cameron Smith is the leading Australian after edging up to two under for the tournament, just three behind Rai, with a third-round 68.
