AUGUSTA, Ga. -- Brains and brawn share the top spot on the leaderboard heading into the second round of the 83rd Masters at Augusta National Golf Club.

Bryson DeChambeau, with a physics degree from Southern Methodist University, fired an opening-round 66, with nine birdies and three bogeys.

Brooks Koepka, who is known to spend as much time in the gym as on the practice green, posted the same 6-under score with a flawless bogey-free round.

DeChambeau birdied six of the last seven holes; Koepka had five birdies on the back nine.

The 25-year-old DeChambeau is No. 6 in the Official World Golf Rankings; 28-year-old Koepka is No. 4. So they head into Friday with it being the first time two top-six players in their 20s share the lead after any round in a major.

DeChambeau nearly aced the par-3 16th hole, which would have been the first of his career, and he hit the pin with his second shot on 18, before settling for a tap-in birdie to close his round.

"I was just looking to shoot something in the 60s because I'd never done that here before," DeChambeau said. "What a magical back nine. The wind started to pick up around Amen Corner, and it was tough. It was not easy one bit."

It was easier for DeChambeau than most. Affectionately known as the "Mad Scientist" for his analytical approach to the game, DeChambeau plays with single-length irons -- each one is 37½ inches long, about the length of a 7-iron -- with club heads that each weigh 278 grams.

Thanks to new PGA Tour rules, DeChambeau played his first round at Augusta National by putting with the flagsticks in. Apparently, he liked the flagsticks' COR (coefficient of restitution), whatever that is.

Last week, DeChambeau spent 14 hours working on a gear system at Dallas National Golf Club, attempting to get on the right side of the spin loft curve with his wedges.

"If I get driven enough to where it's been a long enough time to where we haven't figured something out, I will exhaust every resource until I figure it out," DeChambeau said. "And that's common with me. I become almost a little, in a positive way, OCD about trying to complete something, very positive way."

Koepka, who has won the past two U.S. Open titles as well as the 2018 PGA Championship, came into the week facing questions about his dramatic weight loss.

Koepka didn't play well earlier this season, missing the cut at the Arnold Palmer Invitational, finishing in a tie for 56th at the Players Championship and failing to get out of group play at the WGC-Dell Technologies Match Play.

Koepka admitted his weight loss cost him yardage off the tee, and he said earlier this week that he was on a diet of about 1,800 calories per day. He said he had bloodwork and other medical testing done to figure out what was going on.

"I mean, you're not going to be in the best physical shape at that point," Koepka said. "You look at somebody like Michael Phelps or somebody like that eating 6,000 or 7,000 calories by lunch time. But I wanted to do it and try to lose some weight, and maybe went about it a little too aggressively for just a long period of time and the intensity of what I was doing."

Koepka said his plan was to be physically ready for the Masters, and there was no sign of fatigue on Thursday. He said he's also avoiding the gym, which feels kind of "weird."

"I lift all the time," he said. "I lift too many weights, and I'm too big to play golf. And then when I lose weight, I'm too small. So, I don't know. I don't know what to say. I'm too big and I'm too small.

"Listen, I'm going to make me happy. I don't care what anybody else says. I'm doing it for me, and obviously it seems to work."

Can Phil keep the old magic going?

Phil Mickelson, who is attempting to become the oldest major champion at more than 48 years, enjoyed a drama-free 2-under 34 on the front nine.

But after he made the turn, things took a dramatic turn for the worse. Mickelson hit his tee shot on No. 10 into the woods on the left, punched out and then his third shot rolled off the front of the green. He was able to get up and down for bogey.

Then Mickelson hit his second shot on No. 11 into the water for another bogey. Just like that, he was sitting at even par.

"It looked like after bogeying 10 and 11 that would kill some momentum," Mickelson said. "It was the other way around, because I made two great bogeys that should have and could have been doubles. After going in the water at 11 to hit that close and have an easy bogey and then to make a 6-footer for bogey on 10 after a terrible drive, those were almost momentum maintainers, if you will, that kept me in it."

Mickelson responded with birdies on five of the last seven holes for a 5-under 67, 1 shot behind the co-leaders.

It was his seventh career round at the Masters in which he was under par on all four par-5s. In each previous instance, according to ESPN Stats & Information research, he finished in the top five, including two of his three victories in 2006 and 2010.

"Look, the greens are softer than they've ever been, and they're not as fast as they normally are," Mickelson said. "So [Thursday] was a day to take advantage of it. I'm sure they'll get firmer and faster as the week goes on."

Some big names with work to do

Rory McIlroy came into Augusta as the betting favorite, but he limped through a six-bogey, five-birdie opening round to finish 1-over 73. McIlroy hit only seven of 14 fairways and needed 32 putts through the first 18 holes, leaving him a lot of work to do to get back into the mix on Friday.

"Yeah, it was OK," McIlroy said. "The conditions weren't that difficult. I felt the course was there. It's soft. There's not much wind. I made five birdies, that wasn't the problem. I just made too many mistakes, and that was the problem. And I'm making mistakes from pretty simple positions, just off the side of the green, 17 and 18 being prime examples of that."

Other big-name players who have work to do on Friday to avoid missing the cut: Zach Johnson (2 over), Jordan Spieth (3 over), Justin Rose (3 over), Hideki Matsuyama (3 over), Brandt Snedeker (3 over), Henrik Stenson (3 over), Keegan Bradley (4 over), Fred Couples (6 over) and Paul Casey (9 over).

Players with the 50 lowest scores, including ties, and any player within 10 shots of the second-round leader advance to the weekend at the Masters.

When the key players hit the course

Friday's notable tee times at the Masters Players Starting times (All times ET) Kevin Kisner, Kiradech Aphibarnrat, Shugo Imahira 9:47 a.m. Ian Poluter, Matt Kuchar, Zach Johnson 9:58 a.m. Francesco Molinari, Rafael Cabrera Bello, Tyrrell Hatton 10:09 a.m. Bryson DeChambeau, Dustin Johnson, Jason Day 10:42 a.m. Phil Mickelson, Justin Thomas, Justin Rose 10:53 a.m. Brooks Koepka, Jordan Spieth, Paul Casey 11:04 a.m. Corey Conners, Andrew Landry, Adam Long 11:15 a.m. Justin Harding, Aaron Wise, Angel Cabrera 11:48 a.m. Adam Scott, Hideki Matsuyama, Kyle Stanley 1:05 p.m. Patrick Reed, Webb Simpson, Viktor Hovland (a) 1:16 p.m. Tiger Woods, Jon Rahm, Haotong Li 1:49 p.m. Rory McIlroy, Rickie Fowler, Cameron Smith 2 p.m. (a) denotes an amateur

For the full list of tee times, click here.

Hey, who's that?

There are plenty of recognizable names atop the leaderboard after the first round, but you might have to Google a few of the not-so-familiar ones as you watch the second round.

South African Justin Harding, a first-time Masters invitee, spent most of his first round atop the leaderboard after firing an early 3-under 69.

Harding, who might be recognizable because of his long-handed putter, has won five times since the 2018 Masters -- each of his victories came overseas. He has appeared in two PGA Tour majors, missing the cut at both the 2018 PGA Championship and 2013 Open Championship with a combined score of 12 over.

Harding credits "consistency and becoming a little better mentally" for his improved play.

"[I'm] no longer really going through the highs and lows and the emotional roller coaster, so to speak," Harding said. "But that also comes in not really playing the aggressive mindset game that I was in the past. I'm hitting 3-woods off tees a little more often now, laying back on drivable par-4s, just trying to get a little bit smarter on the golf course, making less mistakes and hoping a couple birdies come."

Canada's Corey Conners was the final player invited to the Masters after winning last week's Valero Texas Open as a Monday qualifier. He flew to Augusta on Valero's private jet Sunday night, took Monday easy and then practiced Tuesday before shopping for clothes.

He was in the first group off on Thursday morning and posted the first 2-under 70, after bogeying 18. He had birdies on Nos. 13 and 14 and eagled No. 15.

"It's been wild, definitely unexpected. But I'm playing some good golf and really excited to be here and honored to be playing in the Masters and just trying to keep riding the good play," Conners said. "I had a great day out there today, a lot of fun, and hit some quality shots and was able to get a couple under par, which was very pleasing."