FARMINGDALE, N.Y. – Dustin Johnson and Brooks Koepka are close friends, and two of the most successful players in golf.

However, Johnson, the world’s No. 1-ranked golfer, knows he has a lot of work to do to catch up to his buddy, a three-time major champion who is running away with the PGA Championship.

Johnson fired a 3-under-par 67 in Friday’s second round at Bethpage Black and stood at 4-under 136 after 36 holes. Coming off a course record 7-under 63 Thursday, Koepka reached 10-under just four holes into his second round, which started after Johnson had completed his day.

Yes, Johnson noticed the 63 during his opening round.

“It’s 72 holes of golf, so I got a long way to go,” he said. “But yeah, teeing off (Thursday), obviously that was a very, very spectacular round. But I wasn’t worried about the 7-under. I was just trying to shoot as low as I could.”

That was a 69, followed by Friday’s round in which Johnson birdied five holes on his first nine but couldn’t quite keep up the pace to make up some ground on Koepka.

“I wasn't thinking anything other than the next shot I had to hit,” he said. “But, you can get it going out here. For me, if I can drive it in the fairways, I can get it going, just need to roll the putter.

“I feel good. I feel like the golf game is in good form. I'm definitely looking forward to a good weekend.”

It remains to be seen whether Koepka and Johnson will be close enough in the standings to play together, but Johnson said he would welcome it.

“We’re very competitive on the golf course, in the gym, whatever it is,” he said. “We’re really good friends. We kind of push each other. So it’s been a lot of fun.”