Brooks Koepka offered a subtle reminder Dustin Johnson has only won one major and said "I like my chances" when asked how he feels about chasing down his fellow American at the US PGA Championship.

On a fascinating moving day at TPC Harding Park, former world number one Johnson climbed into a one-shot lead as a five-under-par 65 left him nine under for the tournament.

But Koepka, aiming to win the tournament for the third straight year, is only two shots back himself as the first major of 2020 nears a thrilling conclusion.

In 2019, it was Johnson who was attempting to reel in Koepka and with the roles reversed the latter did not pass up the chance to lay the battle lines.

"I mean, I like my chances. When I've been in this position before, I've capitalised," Koepka told a news conference. 

"I don't know, he's only won one. I'm playing good. I don't know, we'll see."

Johnson is one of three players inside the top 10 with one major to their name, with Justin Rose and Jason Day also in contention ahead of Sunday's final round.

Koepka, who shot a one-under 69 on Saturday, has four to his name and spoke about the challenges of winning a second major.

"Well, if you look at the top of the leaderboard, I'd say yes [it is more difficult to win a second major]," he added.

"I think expectations. I guess it does become difficult if you think you've played good enough to win multiple ones. 

"But you've just got to keep putting yourself there. I'm doing a good job of that. But the second one definitely is a little bit tougher, I think, as you can see from the top of the leaderboard.

"I'm playing good so I like my chances. Just put the ball in the fairway a few more times and not in the semi. 

"That would be all right, just not short-side myself. If I can do that tomorrow and not short-side myself, I'll have a good chance."