(Reuters) - Daniel Berger stormed to the top of the FedEx St. Jude Classic leaderboard with the lowest score of the week, a six-under-par 64 in Friday's second round, though the ominous figure of golfing heavyweight Phil Mickelson was in hot pursuit.

American Berger, the PGA Tour's rookie of the year last season after recording two second placings among his 13 top-25 finishes, jumped three shots clear of his closest pursuers at the TPC Southwind in Memphis with a nine-under halfway total of 131.

Little-known American Tom Hoge (69) was alone in second, with Mickelson, a five-times major winner who next week will head to Oakmont searching for a first U.S. Open victory after a record six runner-up spots, joint third on five-under after a 65.

"I am putting well, no question," Mickelson told PGA Tour radio. "I didn't strike it as well as I have been this year for the first couple of days. It's just a fraction off and I'm going to go work on it now.

"Hopefully I can figure out what the difference is because if I can hit a few more greens, the way I am putting, I should have a good chance (of winning)."

Equal third with Mickelson were fellow Americans Brooks Koepka (65) and long-hitting Dustin Johnson, who got to eight under before dropping three shots in his last two holes for a 69.

Johnson, the 2012 champion, bogeyed the 17th after bunkering his approach before running up a double at the last, where he three-putted from seven feet.

"I played a lot better than my score," said Johnson, a nine-times winner on the PGA Tour. "Making bogey and double on the last two holes from the middle of the fairway is not a very good finish. I just made a couple bad swings.

"I feel like I am playing really well, I got a lot of confidence in my game on this golf course, I just need to go out and shoot another good score tomorrow."

Berger, whose father Jay was head coach of the 2012 U.S. Olympic tennis team, was delighted to go bogey-free after scrambling well while relying on a red-hot putter.

"My speed control has been great and on the greens, I feel like I am just at home," said the 23-year-old from Florida. "I love the Bermuda (grass) and I don't even read grain here.

"I just look at it and I see it, it's just from years and years of putting on it. Overall just excited to play a bogey-free round."

(Reporting by Mark Lamport-Stokes in Los Angeles; Editing by Andrew Both)