Clark had to play the back nine of his opening round Friday, then got only a brief break before a bogey-free 6-under 64 in a second round in which he didn't miss a fairway or a green.
'It certainly makes it easier to get around,' said Clark, who at 11 under had a one-stroke lead over Arron Oberholser (66) in the rain-soaked Colonial.
'This could be my last week for a while. I'm going to take some time off to heal up. I want to end on a good note.'
The South African was among 57 players -- exactly half the 114-player Crowne Plaza Invitational at Colonial field -- who had to finish their first rounds Friday morning and then go right back out for 18 more holes.
The 36-hole cut, projected to be under par for the first time in the 61 years at Colonial, won't be made until the second round is completed Saturday morning. There were 27 players still on the course when play was suspended Friday because of darkness.
Ben Curtis (67), Jim Furyk (66) and Nathan Green (66) were in at 9-under 131, with Alex Cejka (65), Kevin Na (69) and first-round leader Anthony Kim (69) at 8 under.
Clark's 11-under 129 total was only one shot off the 36-hole tournament record held by Kenny Perry, who opened with rounds of 65 and 63 on way to winning his second Colonial title in 2005.
The first-round scoring average of 69.342 was the lowest at the Colonial in the 25 years since the PGA TOUR starting keeping such statistics. There were 63 rounds under par in the first round, and already 52 more in the suspended second round.
'The golf course is just not playing itself right now,' said Furyk, who at No. 3 in the world ranking is the only of the top 12 players at Colonial.
Instead of firm and fast as usual, rain Thursday and Friday soften things. Balls aren't shooting through the fairways or bounding off sun-baked greens at Hogan's Alley, a traditional tree-lined layout that covers 7,054 yards.
'A little bit slower than they normally are,' Curtis said. 'A little bit softer and they're receptive. That is the reason why the scores are the way they are. This place usually is rock hard and firm.'
The wet grass did make the heavy rough tougher, which Furyk found at the hard way.
On the 611-yard 11th while finishing his first round Friday, Furyk hit his tee shot into the left rough, then pulled a 5-iron that didn't get back in the fairway on the course's longest hole. He then had to hack a shot out of 'an awful lie' and three-putted to finish with a double bogey.
But he rebounded with a quick birdie at the 417-yard 12th hole.
'I hit a wedge in there close,' Furyk said. 'It took a little sting out of it.'
Clark has tried to play through the muscular and joint stiffness in his neck that has bothered him on and off since last summer.
This is only his eighth PGA TOUR event this year, and the South African's best finish was 13th at the Masters, where he was tied for the 36-hole lead. He was better than 62nd only one other tournament, withdrew from the Wachovia and had a closing 81 at THE PLAYERS Championship.
'That wasn't any fun at all,' Clark said. 'I'm not doing myself any favors, not doing my confidence much good either. ... It's time for me to really get it sorted out'
But it will have to wait until at least after this weekend, though he hasn't completely ruled out playing in the Memorial next week.
Clark felt good Friday after 11 birdies with only one bogey in his 27 holes, and never noticed the neck. After shooting under 70 only once in his first 20 rounds this season, he finished two sub-70 rounds in one day.
'I certainly wasn't expecting it,' said Clark, who has never won on tour. 'The adrenaline was going at the end, and I almost had to calm myself down a bit.'
After missing the last two cuts, and six of nine this season, Cejka was 'a little bit scared' coming to the tight Colonial. But his only two bogeys so far came on the last four holes, to go along with a closing birdie.
'I just somehow managed to play well 36 holes,' Cejka said. 'Sometimes you don't expect it.'
Defending champion Tim Herron finished with three straight birdies in a round of 68 to get to 5 under. ... Brian Gay made a hole-in-one Friday at the 194-yard eighth hole with a 7-iron during his first round. But he had six bogeys and one birdie in his final 28 holes Friday and will miss the cut at 3 over. ... Brad Faxon withdrew after an opening 78. The only first-round score worse than that was an 86 by Rod Curl, the 1974 Colonial champ who is invited annually as a past winner. ... Shigeki Maruyama withdrew after nine holes in the second round with a leg injury.
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