Phil Mickelson is one shot off the lead after a 67 at Doral

Two 37-year-old relative unknowns were at the top of the leaderboard on the first day of the WGC-Cadillac Championship, while several of the world’s top golfers struggled Thursday on the Blue Monster.

Australian Marcus Fraser, who is ranked 58th in the world, and American Scott Piercy, ranked 48th, shot six-under-par 66s to share the lead on a cloudy but relatively calm day at Trump National Doral.

Phil Mickelson, at 45 the oldest player in the 66-player international field, was third with a 67. Adam Scott, the winner of last week’s Honda Classic, was in a four-way tie for fourth at 68.

The marquee group of No. 1 Jordan Spieth, No. 2 Jason Day and No. 3 Rory McIlroy were a combined four under par for the day, with Spieth the leader of that pack at three-under 69, good for a three-way tie for eighth.

McIlroy shot 71 using his new cross-handed putting grip and was tied for 16th with 11 others. Day shot a 72 and was tied for 28th, along with defending champion Dustin Johnson and seven others.

Sergio Garcia was one over, Justin Rose three over and Patrick Reed five over in the 66-player field.

Piercy credited his new swing instructor Jeff Smith and some hard work earlier this week for his much-improved play.

“We put in a lot of time Monday, Tuesday, Wednesday and I felt like I got it back at least on the right tracks to see results this week,” said Piercy, a three-time PGA Tour winner. “I was pleasantly surprised.”

Piercy birdied four of the first five holes on the front nine and went out in five-under 31. Birdies at 10 and 11 got him to seven under, but he had two bogeys and a birdie over his final five holes.

“I hit balls for about eight hours on Monday, so it definitely got going in the right direction,” Piercy said. . “Once you get on the golf course, you kind of have to trust it. And I did a pretty good job of that through about 12 holes, then I kind of got a little loose.”

Piercy said he knew it was going to be a good day when he hit a four-iron to 14 feet on the 225-yard fourth hole and made the putt.

“After I made birdie, I was like, all right, swing feels pretty good. I’ve got a three under through four. Let’s see what we can do, and I just continued to hit good shot after good shot,” he said.

“I knew I had tough holes ahead of me, but this place you can’t really get ahead of yourself. You have tough holes everywhere, and you’ve got to play solid.”

Fraser, an Australian, got off to a solid start on the back nine with birdies at 10, 11 and 14. He added four more birdies on his front nine to get to seven under before making bogey at the par-five eighth, where his second shot from the rough went into the water. After taking a drop, he hit his fourth shot on the green and two-putted.

“It was really steady. I felt like I had a lot of control over the ball,” said Fraser, who averaged 267 yards off the tee. “A couple of weak drives, but other than that, I felt like I really gave myself a chance on just about every hole, which obviously you need to do on this golf course. I’m probably one of the shortest hitters here, so I have to play the golf course fairly smartly.”

Mickelson said he putted better Thursday than he did last week at the Honda Classic, where he opened with a 69 and ended up tied for 37th.

Starting on the back nine, he had three birdies and two bogeys, then came in with four birdies on the front nine.

“I hit a lot of good shots, and made some good putts, and just drove it well,” Mickelson said. “It was a good day.

“I think it’s a key round because I knew I’ve been playing well, and the only place I struggled last week was on the greens, reading them.

“Getting off to a five-under-par round today allows me to just go out and play the way I’ve been playing and not have to try to press the issue and go really low to get in contention for the weekend.”