Forget Justin Bieber, it was Michael Phelps who grabbed some of the spotlight at the US PGA Championship as Quail Hollow's speedy greens came into question.

Bieber was spotted at the Charlotte course earlier in the week, but it was Olympic great Phelps' turn, as he tracked the marquee early group of Jordan Spieth, Brooks Koepka and Sergio Garcia.

Only Koepka managed to produce anything of note from that group, his three-under 68 leaving him a shot off co-leaders Thorbjorn Olesen and Kevin Kisner.

Meanwhile, the fast greens at Quail Hollow came into focus on a warm, humid yet relatively calm day in North Carolina.

GREATS GREAT FRIENDS

Phelps, the 23-time Olympic gold medallist, was a keen onlooker as Spieth's group went around on Thursday.

Spieth is bidding to become the youngest player to win the career Grand Slam, and the 24-year-old revealed how close he was to Phelps after his opening-round 72

"It was great. He had followed us quite a few rounds. He's become a good friend and mentor," he said.

"It's pretty awesome to have a mentor like that. He texted earlier in the week. I think he was here for an outing and said he was going out to be out following today."

Spieth added: "We've had dinner, talked through a lot of things that I will probably just keep to myself."

NEED FOR SPEED

The speed of the greens at Quail Hollow became a talking point on the opening day.

U.S. Open champion Koepka described the greens as the fastest he has ever played after his round.

"I think they could slow them up a little bit, I really do. It depends," he said.

"I think we're supposed to have rain tomorrow afternoon, right, 50 per cent chance is what I was told. So that could be a little bit of a factor. That could slow them up and make them a little more receptive.

"You know, we'll see. But they definitely don't need to be faster."

Others are enjoying the challenge far more, with co-leader Kisner comfortable on the new Bermuda grass and Jon Rahm having had plenty of fun.

KOEPKA HITS MARSHAL

Koepka had earlier struck a marshal in the head with his tee shot at the 16th hole.

The man was seemingly still in good spirits, telling the American he "got a good break" after his ball bounced onto the fairway.

Koepka, who said he felt "terrible" for the incident, made bogey anyway, but he ended up shooting a fine round to be just a shot off the lead.