Miguel Angel Jimenez, who briefly held the lead in the third round, fired a five-under 67 to move into second place at 13-under-par 203. Richard Green, Simon Yates and David Lynn share third place at 12-under-par 204. Ian Poulter and David Smail are one stroke further back at minus-11.
Bjorn battled throughout the round, hitting just six of 14 fairways and 11 of 18 greens in regulation. He missed the fairway at the first two holes, but managed to save par both times.
The two-time Ryder Cupper got up-and-down for par at the par-3 third and two-putted for par at the next. He finally got on the board with a six-foot birdie putt at the par-5 fifth at Alpine Golf and Country Club.
He two-putted for par on each of the next two holes. Jimenez, playing two groups ahead of Bjorn, tied him for the lead with a birdie at the seventh. Bjorn reclaimed the lead when he sank an 18-footer for birdie at the ninth.
Bjorn scrambled to up-and-down pars at Nos. 10 and 11 before a two-putt par at the 12th. He stumbled to his first and only bogey of the day at the 13th. Meanwhile, Jimenez birdied the 14th to move to minus-14, giving him a one-shot lead.
Jimenez stumbled to a bogey at the next, while Bjorn parred three straight holes as the duo shared the lead at 13-under. Jimenez closed with three consecutive pars of his own to finish at minus-13.
Bjorn was not done though. After narrowly missing a birdie putt at the 16th, he two-putted for birdie at the par-5 17th to reclaim a one-stroke lead. He lipped out his birdie try at the last to remain one ahead of the field.
'I've got to be satisfied to be leading the tournament,' said Bjorn, a seven- time winner on the European Tour. 'It was tough day, probably the hottest I've ever been on a golf course. It's tough when you come out after eight weeks and all of a sudden find yourself in this situation. I was 5-under par for the first seven holes on day one and have been leading the tournament pretty much ever since.'
The combination of his long lead, as well as the sweltering heat and humidity has worn on the Dane as well.
'That takes a little bit extra out of you,' Bjorn said. 'The heat just makes it a little bit difficult but I'm happy with the way I'm performing. I need a good rest tonight, that's for sure, and then I'll be ready for tomorrow.'
Jimenez was hot early in his round. He birdied three consecutive holes from the second to get to 11 under. The Spaniard notched back-to-back birdies from the sixth to grab his first share of the lead.
However, he cooled off considerably from there. He managed to par the next six holes before his birdie at the 14th and bogey at 15.
'You can't think about targets. It's about today and tomorrow and then next week and if things go well, the big targets take care of themselves,' said Jimenez, about the prospects of gaining a spot on the European Ryder Cup Team. 'My plan tomorrow is that I must give all I have, to play well and let things happen. You cannot do anything more. Of course I would like to get back onto the Ryder Cup Team but time will tell the answer.'
Ernie Els, the defending champion, remains in contention after shooting a 3-under 69. He is at 10-under-par 206 alongside Scott Gardiner, who also shot a 69, Marcus Fraser and Yong-Eun Yang, who each carded 6 under rounds of 66, as well as Raphael Jacquelin, Thongchai Jaidee and Adam Scott, who each posted rounds of 5-under 67.
Barry Lane, Colin Montgomerie and 2003 PGA Championship Shaun Micheel each notched rounds of 5-under 67. That trio stands at 9-under-par 207, and is joined there by Nick Faldo (72) and Miles Tunnicliff (68).