Karlsson started one stroke behind first-round leader David Mathis and carded six birdies and a bogey to reach 9 under. The Swede, who lost here a year ago in a playoff with Lee Westwood, has played his first six career rounds at TPC Southwind under par with this his lowest score yet.
Colt Knost (68) and Keegan Bradley (67) were tied for second. Bradley, the Byron Nelson Championship winner two weeks ago, is among a few who can earn a spot next week in the U.S. Open at Congressional by winning his second tour event since the last U.S. Open.
John Merrick (69) was 5 under, and Fredrik Jacobson (65) and Harrison Frazar (65) were another stroke back. Brandt Snedeker, The Heritage winner in April, shot a 66 to top the group at 3 under that included Mathis (72). With 83 making the cut at 2 over, David Toms (5 over) and Sergio Garcia (7 over) were among those who made an early departure for Congressional.
Karlsson came into Memphis ranked 23rd in the world and looking for his first PGA Tour win in his first full season on tour. He has 11 career European Tour wins but his best finish this year was a tie for fifth at Colonial. That was his first top 10 on the PGA Tour since he tied for second here a year ago.
Nobody beat him over 72 holes here in 2010.
Starting at 4 under and teeing off on No. 10, Karlsson birdied his second hole and finished the back side with birdies on three of the final four holes. He hit an 8-iron to 8 feet on the par-3 11th to set up his first birdie. He saved par on Nos. 12-14 and then hit a wedge from 106 yards to 7 feet to take advantage of a front pin position on the par-4 15th to start his birdie string.
Karlsson found the rough with his tee shot on the par-5 16th but he recovered by hitting a wedge from 124 yards to 5 feet for a second straight birdie. He stuck a 7-iron from 175 yards to 4 feet on the par-4 18th to reach 8 under.
He made the turn and birdied the par-4 first after hitting his second shot within 4 feet of the pin despite being 124 yards out in the primary rough. When Karlsson rolled in a 14-footer on No. 6 for birdie, he became the first player to reach double digits below par at 10 under.
Karlsson dropped a shot with his lone bogey of the round and just his second this week when he three-putted from 34 feet on the par-3 eighth.
DIVOTS: Michael Bradley eagled the par-5 16th when he holed out from 66 feet from a greenside bunker. … Kip Henley didn’t miss out on a big check by deciding to play here. Usually Brian Gay’s caddie, he chose to play instead of work after earning a spot by winning a Tennessee tournament last August. Henley missed the cut at 20 over, while Gay also missed the cut after a 2 over. … Patrick Reed’s pro debut after helping Augusta State repeat as NCAA champs didn’t go too well. He missed the cut at 6 over.