Phil Mickelson, Brooks Koepka one off the lead at the FedEx St. Jude Classic

Before the marquee U.S. Open groupings were announced on Thursday, two marquee players had already gotten their Open prep off on the right foot at the FedEx St. Jude Classic. Brooks Koepka, the reigning U.S. Open champion, and Phil Mickelson, who looks to complete the career grand slam at Shinnecock Hills, each carded four-under 66s at TPC Southwind in Memphis, Tenn., putting them one back of leader Seamus Power.

Mickelson, who has twice finished runner-up in this event, entered the week in good form, having played his last three rounds in 12-under at the Memorial Tournament to finished in a tie for 13th. If not for an opening-round 74 that included some late-round struggles, he may have been firmly in the mix on Sunday at Muirfield Village. A week later, Mickelson made sure he didn't let one get away this time.

"I finished the round off," said the 47-year-old. "I make a bogey (on No. 5, his 14th), and I'm almost making another one, and I was able to gather myself and start to control my thoughts a little bit better and I made a good par putt on the sixth hole and then got another birdie coming in (at No. 8). So I was able to finish the round off even though I didn't quite have it there at the end."

After making just one birdie on his front nine at the par-4 15th, Mickelson quickly picked up a birdie at the par-4 second and followed with an eagle at the par-5 third. A 344-yard bomb down the fairway left him just 207 yards in, and from there he put it to 11 feet and converted. That, plus his final birdie at the eighth to get that shot back from a bogey at the fifth, gave him his best opening round of the season.

Like Mickelson, Koepka has a solid track record at TPC Southwind, having also finished runner-up in 2016 and T-3 in 2015. Those positive feelings, plus a strong day on the greens, where he was able to score despite hitting just 13 of them, helped propel Koepka to the top of the leader board without his best stuff.

"Putted really well, felt like I scored really well," said Koepka, who made six birdies against two bogeys on the day. "I didn't play well at all, to be honest with you. We hit some pretty poor shots, we were kind of all over the place, but, hopefully we'll figure this out for the rest of the week. Love this place, got good memories here. I like the way it sets up from the tee shots, a lot of them suit me, and when you get on good greens like this, good bermuda, I'm a good putter as it is. I grew up on bermuda, you just roll it and it goes in. They're great, I just need to figure out everything else."

If he does figure out everything else, you can throw his name into the mix when it comes to the favorites at Shinnecock. But he's not looking ahead quite yet.

"I'm not looking forward, anything towards next week," he said. "It all depends on what the shot is here, I mean there's nothing better than winning a golf tournament going into a major. Feel like I'm playing well enough to do that."

Ireland's Power leads the way after posting a five-under 65 with just one bogey. It matches his low round of the year, which came in the third round of the CareerBuilder Challenge in January.

Also at four under is 51-year-old Steve Stricker, who is making just his eighth PGA Tour start this season. Next week the 12-time tour winner will play in his 21st U.S. Open, where he'll look to extend his consecutive made cuts in majors streak to 26.

At three under through eight holes, Dustin Johnson looked to go low on Thursday afternoon, but a double bogey at the ninth dropped him back to one under. He was able to salvage his round though, collecting birdies at the 12th and 16th holes, the first of which he chipped in with his one foot submerged in the water. He finished with a three-under 67 and is two back.