Stricker started the day a further two shots adrift in a tie for eighth place in a group also including Adam Scott of Australia, Argentina’s Angel Cabrera and Swedish veteran Jesper Parnevik.

Mahan’s late start allowed the early risers to make inroads into his lead and Stricker, a fellow American Ryder Cup hopeful, was eager to give chase.

Stricker, who won the 2007 event at Westchester Country Club, holed a birdie at the par-three second hole, and then eagled the par-five third, chipping in from 67 yards form the fairway.

Stricker then closed to within two shots of Mahan with a birdie at the fifth hole to reach the turn in 31, four under for his round.

He added another birdie at the 11th and drew level with the leader at the 13th with his fourth birdie of the day.

The outright lead became his with a birdie two at the 15th to move to 10 under on the way to a 64 before Mahan had begun his round.

The low scoring has been a surprise given the pre-tournament predictions of tight fairways, punitive rough and small greens but Stricker said he expected even lower numbers.

“It is a good course. You’ve got to do a lot of good things,” Stricker said.

“The greens are tough. But I didn’t think the scoring was going to be this low. And when Hunter goes out and shoots that nine-under round yesterday, it shows you that it is capable and it is out there.

“I think the course is in such good shape, that’s why the scoring is good, and I think as we get further along into this tournament, the scoring could get a little bit better just because guys will find a way to play it, understand it a little bit more and feel a little bit more comfortable with what they are doing out there.”

Mahan fell a further shot behind Stricker when he teed off on the 10th hole, going out in 37, but he birdied the first, his 10th, and was nine under with seven holes to play.

American Dudley Hart, who started the day tied for third at four under, also improved his position, with birdies at the fourth and eighth holes before a bogey five at the ninth kept him at five under.

An up and down back nine with two birdies and two bogeys kept Hart there until a birdie at the 18th sent him to six under with a second-round 69, four behind Stricker.

Former US Open champion Cabrera will also start the weekend at six under following his 67 today, the Argentinian producing a bogey-free round of four under par.

And Kevin Streelman, tied for third overnight on four under, began his second round brightly with birdies at the fourth and sixth to move to six under after seven holes.

Anthony Kim quickly got over an opening day he described as “one of the toughest rounds I’ve ever played” to move to six under after 14 holes before finishing at five under at the halfway mark.

Kim, a two-time winner in just his second year on the PGA Tour and an automatic qualifier for the US Ryder Cup team, had been stung by a bee and suffered an adverse reaction to medication during his opening round of 70.

Today, Kim provided the bite with five birdies, his only blemish coming at the par-three eighth, his 17th of the day, which he bogeyed for a second-round 67.

Sergio Garcia also shot a 67 to send him to five under at the halfway stage. Starting on the 10th tee, the US PGA Championship runner-up bounced back from a bogey at his second hole with birdies at the third, fourth, eighth and ninth to reach the turn in 33, four under for the tournament.

He too bogeyed his 10th hole but moved to five under with birdies on his 13th and 16th holes.

Casey suffered an early setback to his second round with a bogey at the 12th, his third hole of the day, before returning with a birdie four holes later and another at his 10th of the day to stand at six under with four holes to play.

Kenny Perry, the FedEx Cup points leader in Tiger Woods’ continued absence due to injury, was also making his move, sinking four birdies on his outward nine and was six under for the tournament with seven holes to play.