He still matched his career low with 9-under-par 62 on soft and vulnerable TPC Boston, giving him a one-shot lead over Mike Weir of Canada at the halfway point of the PGA Tour's second playoff event.
"It just was strange," Clark said. "I felt good all day and just kept making birdie after birdie after birdie. And then on the front nine, a couple of eagles really boosted the round. For a while there, there certainly was a chance of 59."
On another day of lowing scoring, Clark was trying to get into contention when he shot to the top of the leaderboard by playing a six-hole stretch in 7-under, including two eagles. After a wedge to 3 feet on the par-5 seventh put the South African at 11-under for the round with two holes to play, it all fell apart.
Clark missed the green on the par-3 eighth and failed to convert an 8-foot par putt.
"That was the end of the dream," Clark said.
He missed the ninth fairway, watched his approach bound through the green and he missed his 10-foot par putt for the 62, putting him at 14-under 128.
Weir, who tied the course record Friday with 61, took a while to warm up until he settled in for 68.
He prefers a stiffer test, but the former Masters Tournament champion has won events such as the Bob Hope Classic.
"I'm going to have to do a little better," Weir said.
Vijay Singh, who won The Barclays in a playoff last week to lead the FedEx Cup standings, picked up three birdies on the final four holes for 66 and was two shots back at 12-under 130, along with former British Open champion Ben Curtis, who had 65.
Jim Furyk (65), Sergio Garcia (64) and Ernie Els (65) were in the group at 131 that also included Kevin Streelman, who is turning into this year's poster boy of how playing well can pay off big in this points format.
Streelman was No. 102 when the playoffs began, tied for fourth at The Barclays to move up to No. 37, and is continuing a strong run that began after the U.S. Open.
The PGA Tour rookie from Chicago is 78-under par in his past eight tournaments, plus these two rounds.
"We all get on streaks where you feel like things are going well, and you try to ride it as long as possible," Streelman said. "And I feel like I've been playing some good golf."
He is assured of making the third round next week in St. Louis for the top 70 in the standings.
Augusta native Charles Howell needs a strong weekend to make it to St. Louis. Howell, who is 104th in the standings, shot 66-70--136 and is tied for 34th place.
Former Louisville, Ga., resident Brian Gay is also tied for 34th place in the tournament after rounds of 68-68. Gay is 63rd on the FedEx Cup points list.