Top-ranked Jon Rahm showed no trace of the magical form he used to win the Masters, struggling to a six-over par 76 in Thursday's opening round of the PGA Championship.

The 28-year-old Spaniard stumbled in with two birdies, six bogeys and a double bogey at Oak Hill to stand 10 strokes adrift of clubhouse leader Bryson DeChambeau.

Rahm has won four PGA Tour titles this year and captured the green jacket last month at Augusta National for his second major title, enduring a 30-hole marathon on what would have been the 66th birthday of his idol, the late Spanish legend Seve Ballesteros.

But at Oak Hill, Rahm's 76 matched his worst-ever round at a PGA Championship from last year's third round at Southern Hills, when he shared 48th.

Rahm also fired the worst opening round by a world No. 1 at the PGA Championship, his six-over breaking the mark of five-over shared by Tiger Woods in 2005 and Greg Norman in 1990.

In picking apart his disaster day, Rahm started with reaching only five of 14 fairways and just seven of 18 greens in regulation.

"The main thing on this course is hitting the fairway," Rahm said. "If you put the ball in the fairway you can actually give yourself a lot of good chances… and that's what I didn't do."

Rahm sank a 16-foot birdie putt at the 10th hole, his first of the day, and followed with five pars.

"The first six or seven holes I played really good. Put myself in a good spot," Rahm said. "And after that I found myself battling.

"Couldn't find the fairway and the fairways that I missed cost me bogeys."

He made three bogeys in a row starting at the 16th, two more at the second and par-3 third and another at the sixth before a double bogey at seven, where he needed three shots just to find the fairway.

Rahm saw hopeful signs in missed par putts of five feet at the third, four feet at the fourth and seventh.

"The only thing I can look back on myself is the three short putts I missed on (my) back nine," Rahm said. "I'm between three to five feet. If I make those putts, I shoot 3-over which is not the worst-case scenario."

Rahm finished with an eight-foot birdie putt at the eighth and a four-footer for par at the ninth.

"It wasn't my best swings on the last two holes and made a birdie and a par, so there's many ways to do this," Rahm said. "You don't need to play perfect."

Rahm expressed optimism at being able to make the long climb back, even though just making the cut appeared to be a difficult task.

"If I can somehow manage to putt a low one tomorrow and find myself close to even par going into Sunday, I think I'll have a decent chance," Rahm said.