Karlsson, who has 11 career European Tour titles but is playing the PGA Tour full-time this year, lost a four-hole playoff to Lee Westwood at the TPC Southwind a year ago.
He is 9 under heading into the weekend - his first 36-hole lead on tour - holding off Americans Keegan Bradley and Colt Knost at six under.
He said it is a bonus he did well here last year, but added: “This is a new year, and even now most likely going to be in the lead after two days makes no difference.
“We've come halfway, and who knows? I might finish 55th.
You never know. You've got to keep going, especially on punishing golf courses like this.” Bradley is back on his game after needing a few days to deal with the demands of winning his first PGA Tour title.
The tour rookie missed the cut a week ago at Memorial days after winning the Byron Nelson Championship. He shot a second straight 3-under 67 on Friday in a bogey-free round with three birdies.
“Last week was kind of a tough week for me,” Bradley said. “I was so tired. It took me by surprise how many more things I needed to do, but this week I'm back to normal. Me and my caddie were out there doing what we did at Nelson. We feel good again.” Bradley won the Byron Nelson for his first professional title just nine days before turning 25. Now the man who never won on the Nationwide Tour is in position to make a run at a second win in three weeks, which would punch his ticket to the US Open at Congressional.
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.
The last player to lead by three strokes after 36 holes here was Justin Leonard in 2005. He went on to win the tournament.
Nobody beat Karlsson over 72 holes here in 2010, and he credits the lessons learned on his first practice round here a year ago: Avoid hitting into the rough on the 7,239-yard course as much as possible to better set up shots into the greens. Temperatures in the high 90s the past couple weeks have firmed up the Champion Bermuda greens.
“It can be better to have an 8-iron from the fairway or a 7-iron from the fairway instead of a wedge from the rough,” Karlsson said. “The greens are firm, and the rough is really, really brutal to control the golf ball out of. You can hit it. You can hit it really far, very, very far because you can get some vicious flyers.” 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 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.
“I hit a bunch of good iron shots. It was nice obviously to get it going. As you said, it's not an easy golf course.
You just don't walk out there and shoot 68, 67. It's always nice to get off to a good start. I played very well on my back nine as well. Gave myself a bunch of chances, and it was good,” Karlsson said.
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.
Knost is among the Nationwide Tour graduates from 2010 playing well here. The SMU graduate who turned pro in 2007 has made the cut in eight of his 16 PGA events this year and in good position for his best finish yet.
“Kind of been struggling coming into this event, but you know I just kind of took a new mindset this week and just got off to a great start yesterday and played solid again today,” Knost said.
