But while Casey and Mahan were in the majority in struggling in difficult conditions, each carding a three-over round of 73, Sabbatini recorded a second consecutive 67 of the week.

He was one of only 11 players to shoot under par in humid, hot conditions that peaked at 90 degrees with a breeze that started out light but caused problems in spells throughout the day.

The South African made the turn in 34, with birdies at the first and fifth holes before a bogey at the sixth. Another one stalled Sabbatini at the 14th but he bounced straight back with birdies at 15, 16 and 17 before closing with par to leave him at six under for the tournament.

His performance even drew praise from Tiger Woods, whom Sabbatini has in the past taunted as being "as beatable as ever".

Woods said: "(It was) great scoring considering the wind was up when he played yesterday and then came up towards probably the middle of his round today.

"So Rory played beautiful to shoot that."

"It's a matter of picking and choosing your targets out there," Sabbatini explained earlier in the day.

"There are just opportunities that you have that you can be aggressive but you have to kind of back down and just give the course respect, otherwise it'll hit you pretty quickly.

"I didn't quite hit the ball as well as I did yesterday. I hit a couple more drives today in that that hay they call rough out there but it was a little more of a grind today, a little more work.

"I think the positive thing I can take out of it is that I did battle out there today and came out on top."

Sabbatini will start the third round on Saturday playing alongside Masters champion Zach Johnson, who shot the lowest round of the day with a five-under 65, a score later matched by Davis Love III to take the former US PGA champion to one under.

Woods, Scott Verplank and Kenny Perry are tied for third place on two under with five-time event winner Woods carding a 70, starting his round with a bogey and a birdie, and adding another of each on his front nine.

"It was playing tricky," Woods said. "The fairways were quick, a lot of the pins you couldn't fire at.

"I just had a hard time making the adjustment."

Casey had a day-long battle to stay in touch and finished the day at level par for the tournament, six shots adrift of Sabbatini. Three bogeys and three birdies on the front nine set the trend and a birdie at 11th was followed by a bogey five at the 13th before a birdie at the par-three 15th sent him back to three under.

Then it fell apart again for the Ryder Cup star as Casey bogeyed the 16th and double-bogeyed the 18th for a disappointing end to a tough day.

"I struggled a bit today and found the rough and when you find the rough around this golf course it's very penal and tough to recover from," Casey said.

"However, we carry on. I'm even par and not completely out of this thing yet. If I get it going over the weekend we will see what happens."

Mahan went out in the afternoon wave of starters and like Casey he shot a 73 to leave him at level par for the tournament.

Lee Westwood had an inauspicious start to his second round, bogeying his first two holes having teed off at the 10th hole. The Englishman had started the day one off the overnight lead at two under in a group of six players that also included five-time event winner Tiger Woods.

Westwood bogeyed at 10 and 11 but steadied himself with a run of six pars before birdies at the 18th, first and third took him to within a shot of Sabbatini. He then carded bogeys at the fourth and sixth to end the day with a 71 at one under for the tournament.

Two bogeys and a birdie saw Colin Montgomerie slip to three over par for the tournament but Sergio Garcia had a terrible outing.

The Spaniard, playing his first tournament since blowing his third-round lead at the Open at Carnoustie two weeks ago, started the day at one over and birdied his first hole at the 10th.

A bogey at the 16th, though, was followed by a quadruple-bogey eight at the 18th and Garcia began his inward nine with a run of bogey, birdie, birdie, bogey before a double bogey at the sixth left him at eight over for the tournament with a round of 77.

Canada's Mike Weir withdrew from the tournament citing a neck injury having played 10 holes of his second round. His withdrawal reduced the field to 82 players, Jim Furyk having pulled out before the first round began on Thursday with an upper back injury.