SAN FRANCISCO – Mellow Dustin Johnson doesn’t sweat much in life.

Not even losing his yardage book during a major championship.

“It’s got to be in the bottom of my bag but I didn’t want to take all my clubs out on the golf course,” Johnson said after posting a 5-under-par 65 on a cool and crisp Saturday at TPC Harding Park to grab the lead in the 102nd PGA Championship through 54 holes.

Many players would have freaked out without a yardage book in their back pocket. Then again, Johnson isn’t most players. The guy whose heart rarely races loves to kick back and float on a boat and just plain relax from time to time. So when he couldn’t find the book full of notes and the X’s and O’s of the golf course, he turned to his brother, A.J., who is his caddie.

“We were fine,” Johnson said. “I just used a regular yardage book. I use it more so I can get the yardages out of the fairway and where the flag is. But my brother had an extra one, so it was perfect.”

He could have said the same of his putting stroke. The world No. 5, who won The Travelers Championship in June and is one of the longest hitters in the game, used the shortest club in his bag to make hay. In assuming residence at the top of a loaded and packed leaderboard, Johnson needed just 24 putts to make 140 feet of putts in his round that featured eight birdies. And he leads the field in Strokes Gained: Putting. With rounds of 69-67-65, he’s at 9 under and leads Scottie Scheffler (65) and Cameron Champ (67) by one shot.

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“I putted really well. That was key,” said Johnson, a winner of 21 PGA Tour titles. “But I hit a lot of good shots to give myself some good looks because the flags are tucked. The greens are firm and fast. So I did hit a lot of quality iron shots.

“Tomorrow, I definitely need to hit some more fairways, because it’s really tough playing this golf course from the rough. The bunkers, too, are very tough.”

Emerging victorious from this leaderboard will be tough, too.

Seventeen players are within four strokes heading into the final 18, with eight of them ranked in the top 20 in the world – including world No. 6 Brooks Koepka, the major master who is trying to become the first to three-peat in this championship since it went to stroke play in 1958.

Koepka, the winner of the 2017 and 2018 U.S. Open and the 2018 and 2019 PGA Championship, birdied two of his final three holes to roar back from a trio of consecutive bogeys earlier in the back nine.

Koepka’s 69 placed him at 7 under along with Collin Morikawa (65) and Paul Casey (68), two shots behind Johnson and one behind Scheffler and Champ.

“I feel very comfortable around the lead in the big events,” Koepka said. “Obviously we don’t have fans here, which makes it a little different when they’re hooting and hollering, which can be fun if they’re cheering for you, but if they’re against you it’s not so much fun. It’s going to feel completely different than any one we’ve ever played.

“I’m looking forward to it tomorrow. It should be a fun shootout. I like my chances. When I’ve been in this position before, I’ve capitalized.”

Three back of Johnson is some serious cream rising to the top – Bryson DeChambeau (66), Tony Finau (67), 2013 U.S. Open champion Justin Rose (70), 2015 PGA Championship winner Jason Day (70), Daniel Berger (70) and Tommy Fleetwood (70).

Four back are Joel Dahmen (68), Si Woo Kim (68), Matthew Wolff (68), Xander Schauffele (69) and 36-hole leader Haotong Lee (73).

“Worked hard to stay in it and obviously a legitimate chance tomorrow,” said Rose, who had three bogeys in his first five holes but weathered that early storm. “Going to have that freedom to go out there and play a few groups ahead and just see if I can find my round where I can kind of get it all together.

“I’ve sort of had good patches, but I haven’t really had a lot confidence in the game yet this week, so I’ve done a great job of giving myself a look. I’m looking for that one round. I’m looking for my round of the week.”

So, too, are many others.

Johnson has experience on his side heading into the final round. He had 11 top-10s in majors before breaking through to win the 2016 U.S. Open. In his last 13 majors, he’s had five top-10s, including runner-up finishes in the Masters and PGA Championship last year.

“I’ve been in the hunt a bunch of times in a major,” he said. “I’ve got one major, so having that experience is definitely going to be beneficial. All I can do is go out and play my game and shoot the best number I can. I’m going to just try and go out and shoot as low as I can tomorrow just like I did today.

“You know, just take what the golf course gives me and just keep on going because, you know, it doesn’t really matter what other guys are doing. I’ve got to hit a lot of fairways and a lot of greens. If I can do that tomorrow, I’m going to have a good chance coming down the stretch on the back nine.”