Ernie Els, who has had a chance at each of the season's first three majors, and Justin Leonard, who finished second to Davis Love III in 1997, are tied for second at 6-under-par 66.
Vijay Singh, the 1998 champion, Briny Baird, Luke Donald and American Ryder Cup hopeful Scott Verplank are tied at 5-under-par 67.
Most reports indicated that Whistling Straits would be one of the most difficult venues in major championship history. The course measured 7,514 yards and with eight holes on the shores of Lake Michigan, a heavy wind could cause scores to be very high.
As the players greeted Whistling Straits Thursday morning, tees were moved up and the course was shortened by almost 200 yards. The wind was relatively calm despite lower temperatures that forced most players to don a sweater.
Even though Whistling Straits, hosting its first major, yielded low scores, not everyone took advantage.
Tiger Woods, a two-time former champion who has not won a major since the 2002 U.S. Open, struggled to an opening-round, 3-over-par 75.
Woods began on the 10th tee and flew out with a birdie. He ran into trouble immediately after that when his drive at the 11th hooked into the rough. Woods could only advance his ball 100 yards into more rough and when he finally reached the green, he three-putted for a double bogey.
Woods, who won in 1999 and 2000, missed a 4-foot par putt at the 12th, then missed the green at the 13th. Woods two-putted from 35 feet for another bogey and a 3-over start through four holes.
He calmed down and made three birdies and three bogeys the rest of the way for his 75, which tied him for 104th. That score matched Woods' highest round at the PGA Championship, which he also posted in the final round of the '97 event at Winged Foot.
'Well, I got off to a nice start and then ran into a little bit of a problem there for a little bit,' said Woods. 'I just didn't hit the ball all that poorly, but I sure putted bad.'
Woods' major streak is at nine and if he is to erase that, he'll have his work cut out for him. He spotted Clarke 10 strokes after one round.
Clarke hit a lob-wedge to 12 feet for a birdie at the first. He missed the green with his second at the par-5 second hole, but lagged his eagle putt to 3 feet and rolled it in for birdie No. 2.
At the third, Clarke knocked an 8-iron to 18 feet to set up birdie. He made it four birdies in a row at four when his 9-iron stopped 12 feet from the flag.
'We had the better conditions this morning,' said Clarke, who is majorless in his career with two good chances at the British Open. 'The wind was blowing a little bit, but not that hard, and the wind eased for us on the back nine. But the greens were soft, and some of the pin positions were, I would not say generous, but reasonable to get at.'
Clarke rattled off two consecutive birdies from the seventh, but dropped his first shot of the championship at No. 9. He pulled a 6-iron into one of the 1,400 bunkers at Whistling Straits, then blasted his third through the green. Clarke chipped to 2 feet and tapped in his bogey putt.
The 35-year-old rebounded nicely from the mishap at nine with back-to-back birdies at 10 and 11, both from inside 10 feet. Clarke mis-hit an 8-iron at the 13th and plugged into a bunker. He could not go at the flag, instead hitting out sideways to the front of the green. Clarke made bogey, but once again wasted little time in atoning for his mistakes.
At the 14th, Clarke hit a pitching-wedge to 5 feet and sank the birdie putt to go one clear of Leonard and Els.
Clarke, a European Tour veteran and three-time Ryder Cupper, knows that there is a lot of golf left on a demanding course.
'I think if the wind keeps blowing, the greens firm up, then we are going to see a real challenge,' said Clarke. 'Not that it's not now, but if the wind keeps blowing, the greens dry out, then I think there won't be that many low scores this week.'
Els started on the back nine Thursday and was 1 under par through five holes. He drained a pair of 8-footers for birdie at 15 and 16, then ran home a 50-footer for birdie at 17, to make it three in a row and a 4-under-par 32 on the first nine.
The No. 2 player in the world reached the green with a 6-iron at the par-5 second and two-putted for birdie from 45 feet. Els hit a 9-iron at the par-3 third and made the subsequent 5-footer for birdie. He parred out for his 66.
'I wasn't sure what to expect scoring wise today because the practice rounds were pretty difficult,' said Els, who finished second at the Masters and British Open. 'I'm not sure if we expected to shoot 6 under around this course, but that was nice.'
Leonard, the 1997 British Open winner, mixed a birdie and a bogey over his first three holes. He birdied seven and 10, then tallied three in a row from the 12th, including a 25-footer at the 13th.
Leonard came up short of the green with his second at 16. He pitched to 5 feet and made birdie for his 66.
The 32-year-old has struggled in 2004 with one top-10, but credited his group with his first-round success.
'Watching Darren (Clarke) and K.J. (Choi) birdie the first hour that we played, it was fun,' said Leonard. 'I think they were playing so well, I kind of got sucked into it on the back nine. I think we all definitely fed off each other, and it was just a fun day, a fun group to be a part of.'
Choi, Chris DiMarco, 50-year-old Jay Haas, Stephen Ames, Stuart Appleby, Geoff Ogilvy, Tim Petrovic, Loren Roberts and Padraig Harrington are knotted in eighth at minus-4.
Masters champion Phil Mickelson flew out of the gate Thursday afternoon with three birdies in his first three holes. He played even-par the rest of the way for a 69 and shares 17th with Carlos Franco, Paul McGinley, Chris Riley and Duffy Waldorf.
Defending champion Shaun Micheel posted a 5-over 77 and is tied for 129th place.
John Daly, who won this event in 1991 and played with Woods and Singh on Thursday, played poorly. He made an eight on 18 en route to a 9-over-par 81, which is good for a share of last place.