The Dane produced a flawless round, including five birdies on the back nine, to end the day clear of Australia's Richard Green on 19 under par.
Green was the day's big mover as his eight-under-par round, the best of the day, moved him into contention.
Ireland's Damien McGrane is two shots further back, after a round of 68, one shot clear of Daniel Vancsik and England's Robert Coles and Robert Rock, who are all at 13 under.
Another pair of Englishmen are on 12 under in the form of Chris Wood, tipped by many as a possible winner after his fine performance at the recent BMW PGA Championship, and Steve Webster.
Luke Goddard recovered from his 73 yesterday to post a round of 68 that moved him into a share of ninth place with Wales' Bradley Dredge, Denmark's Mark F Haastrup and Scotland's Andrew Coltart.
But it was a poor day for Bjorn's playing partner Carlos Del Moral.
The Spaniard had entered the lead tied for the lead with the two-time Ryder Cup winner, but he made an awful start to his round with bogeys at the first and second.
He recovered to pick up shots on the par fives at six and seven, but then dropped shots at 11 and 15 as he posted a two-over total of 74 to drop to 10 under for the tournament.
Meanwhile, England's Lee Westwood overcame a slow start in gusting winds with birdies on the last three holes to grab a share of the lead with American Garrett Willis in the St Jude Classic second round on Friday.
Westwood tees off today a stroke ahead of the field after opening with a superb seven-under-par 63, had slipped back into the pack after covering the first 15 holes in one over.
In the LPGA State Farm Classic, M.J. Hur had two chip-in birdies in a second consecutive 6-under 66 on Fridat that gave her a one-shot lead over Amy Yang.
Yang was at 11-under 133 after a 65 at Panther Creek Country Club.