Friday's four ball (best ball) format, which takes the lower score of a pairing of two golfers on each hole, is predisposed to the proliferation of lower scores, and indeed, every coupling of golfers competing was under par on the day.
Clarke, of Northern Ireland, and Denmark's Bjorn were a dynamic duo on the first day of the tournament's debut at TPC Wisconsin, with each accounting for seven birdies across 12 different holes.
But they have Australian twosome Richard Green and Mark Hensby in hot pursuit, just two behind them.
"Certainly for me knowing Thomas's game so well made it so much easier for me to go out there and free it up today," Clarke said. "I really enjoy playing with him because I know he's there if I mess up, and likewise I try to be there for him.
"We have, it's like a 'no sorry' policy. We're both trying as hard as we can and we both hit a lot of really good shots today and made the most of our opportunities."
Each got on individual hot streaks, nabbing birdies on four straight holes. Overall, Clarke pocketed birdies on Nos.2, 4-7, 9 and 14. Bjorn had birdies at Nos.4-5, 11 and 15-18.
"I think we were tactically very, very sound today and did the right things when we had to," Bjorn said. "When we had two balls in the fairway or two balls on the green, we could be aggressive and that's why we holed more putts than we probably normally would, and we could take on a little bit more flags."
Seven duos are within four strokes of the leaders.
Green and Hensby at 10-under 61, were good for second place. Hensby had the rougher day of the two with a pair of bogeys, but six birdies by Green kept them in contention.
"I think we just both really enjoy the golf course, and the conditions are beautiful," Green said. "It's a lot of fun to play and hit the right shots required on this golf course. I felt comfortable out there."
Defending champion Ernie Els of South Africa and his partner Tim Herron are tied for third at nine under alongside Thailand's Thongchai Jaidee and Australia's Brendan Jones.
Tied for fifth at eight under are Australians Cameron Percy and David Bransdon, fellow Australians Stuart Appleby and Michael Wright, South Korea's K.J. Choi and Japan's Hiroyuki Fujita and Americans Gene Sauers and Glen Day.
Saturday will switch things up with a scramble format - golfers choose the best lie of the two shoots they take from each location along the hole until the hole is completed - before reverting to best ball format on championship Sunday.
8°C