Phil Mickelson was happy with his form to begin the US PGA Championship as he described the Bethpage Black course as a fair but tough test.

Mickelson shot a one-under 69 to finish the first round tied for ninth alongside the likes of Dustin Johnson and Jordan Spieth.

He had some trouble to start his day as he went to the ninth hole at two over, but he birdied three of his next four holes to move to one under – six shots behind leader Brooks Koepka.

"I let a lot of shots slide the first eight holes and then I made up a lot of shots off the last 10, so the game feels good," Mickelson told TNT after his round.

"The golf course is such a good, fair, hard, tough test but if you play well you saw a couple guys go really low, you can do that out there, it's set up very fair, but if you don't play well you can see a lot of guys go high."

Koepka leads at seven under and Danny Lee sits one shot back, but, after that, Tommy Fleetwood is at three under.

There were a lot of players who finished the day over par as they struggled with the length and difficulty of the course.

"It is a challenging long golf course, and what's cool about it is we're playing major championships on a municipal golf course," Mickelson said.

"And Bethpage 2002 really set the tone in doing that for the first time, I think it's really neat."

Mickelson – whose one US PGA Championship victory came in the tri-state area in 2005 – found himself surrounded by crowds throughout the day and revelled in the atmosphere after his round.

"It's the best," he said.

"It's hard to explain how good it feels to walk up 18 and have that reception, it means a lot to me and I'm very appreciative and I love coming back to this area."