The top seed and defending champion, who had been sidelined by a knee injury after winning his 14th major title at the US Open in June, surrendered an early one-hole advantage in the second round before losing on the 16th green.

Clark, world-ranked 33rd, had not reached the third round in five previous appearances at the event but outclassed the game's leading player with a flawless six-birdie display.

Woods's unexpected departure left the opening World Golf Championships (WGC) event of the year with just one of the top five seeds, fifth-ranked Phil Mickelson having squeezed through for the second day in a row.

Second-seeded Spaniard Sergio Garcia and Padraig Harrington (third) had been eliminated in the opening round while Vijay Singh (fourth) squandered a two-up advantage with two holes remaining to lose to Britain's Luke Donald after 19 holes.

The absence of three-times champion Woods, however, will be sorely felt for the rest of the week at Dove Mountain's Ritz-Carlton Golf Club with spectator attendance and television ratings likely to dip.

Woods, who beat Australian Brendan Jones 3&2 in Wednesday's opening round on his return to the Tour after an eight-month absence, led Clark by one after four holes before his opponent took control in the bright desert sunshine.

The straight-hitting South African piled up five birdies in nine holes from the fifth before sealing victory with a conceded birdie at the par-three 16th.

"I knew if I came out and played good, I'd always have a chance," said Clark, who was hammered 5&4 by Woods in the second round two years ago. "I felt pretty relaxed out there.

"Luckily I've had the experience of playing with him and I knew I had to treat like another round of golf and go out and play. It was a great match for me."

BIRDIE DISPLAY

Woods, beaten for only the fourth time in his last 28 matches at the event, applauded Clark's six-birdie display.

"I just happened to catch Tim playing really well and I didn't make enough birdies to answer him," the 33-year-old American told reporters.

"I hit it really good today, I just didn't make enough birdies. Tim made a lot of birdies out there and I didn't answer in the middle part of the round. Consequently I got behind."

Mickelson, fresh from victory at last week's Northern Trust Open, held off a late fightback by fellow American Zach Johnson to win one up.

"I knew it was going to be a tough match," Mickelson said after being taken to the final hole for the second day in a row. "Zach doesn't make any mistakes, he's an incredible putter and I was fortunate to get hold of a victory."

Mickelson will next meet compatriot Stewart Cink, who won a marathon battle with Britain's Lee Westwood after 23 holes.

Paul Casey charged four ahead of Australian Mathew Goggin after nine holes before cruising into the third round 6&4 while fellow Britain Ross Fisher hammered American Pat Perez 6&5.

American Jim Furyk birdied three of the first five holes en route to a 4&2 win over Germany's Martin Kaymer and South African Ernie Els reached the third round for the first time in six years by beating American Steve Stricker 3&2.