Chad Campbell, Chris Couch, Robert Allenby and Harrison Frazar finished at 5-under-par 66 on the Dubsdread Course at Cog Hill Golf and Country Club as inclement weather delayed the start of the tournament by an hour.
Furyk started on the back side and hit a wedge inside 7 feet for a birdie at the par-four 10th. Furyk added a birdie at the par-5 15th and moved to 3 under with a birdie at the par-4 first.
At the par-4 fifth, Furyk played a 6-iron to 15 feet and drained the birdie try. He kept on rolling at the par-3 sixth and rolled in a long putt for another birdie.
Furyk dropped his approach inside 6 feet for a birdie at the par-4 seventh and landed his second shot within four feet at the par-4 eighth. Furyk converted the short birdie putt to make it four straight and closed with a par at the ninth for his share of first.
'I've just always been real comfortable at Cog Hill,' Furyk said. 'I know if I'm playing well, I can do well on this golf course. If I come in here playing well, I feel good about it.'
Curtis sent a 9-iron within inches of the hole for a tap-in birdie at the first. He tallied a birdie at the par-4 fourth and hit his tee ball to 6 feet for a birdie at the par-3 12th.
The 28-year-old, who has struggled since his breakthrough victory at Royal St. George's, played his tee shot to 20 feet and ran home the birdie putt at the par-3 14th. Curtis then reached the green in two at the par-5 15th and made the eagle putt to jump to minus-6.
Curtis then hit his second shot to 20 feet for a birdie at the par-4 17th and a piece of the first-round lead.
'I've had a few decent tournaments, but it hasn't affected me,' Curtis said of the pressure that comes with being a major champion. 'I think more than anything the stuff that you have to deal with after you win, I think I basically got tired, and this year there's been no excuses. It's like my rookie year again.'
Fischer played the back nine first and jumped out of the gate with three straight birdies from the 10th. He stumbled to a bogey at the 13th but got that shot back with a birdie at the par-4 18th.
The 35-year-old made a move on the front side with four birdies to join Furyk and Curtis atop the leaderboard.
Joe Ogilvie, James Driscoll, Tim Herron, Joey Sindelar, Brian Gay, Robert Gamez and Kent Jones are knotted at 3-under-par 68.
Defending champion Stephen Ames carded a 1-under-par 70 to finish in a large group six shots off the lead.
World No. 1 Tiger Woods, a three-time winner of the Western Open, struggled on Thursday with a pair of bogeys over his last four holes to finish at 2-over- par 73.
'I didn't really play all that well today,' said Woods. 'Didn't putt well, didn't play well, just had a tough day.'
Vijay Singh was one shot better at 1-over-par 72.