Updated

John Cook fired a five-under 66 on Friday to join first-round leader Roger Chapman atop the leaderboard after the second round of the Senior PGA Championship, the first major this season on the Champions Tour.

Cook and Chapman, who bogeyed his last for a four-under 67 on Friday, finished 36 holes at seven-under 135 and are one stroke clear of arguably the best player in Champions Tour history at The Golf Club at Harbor Shores.

Four-time winner Hale Irwin, who, at 66, owns the most Champions Tour victories with 45, carded his age on Friday and moved into third place at five-under 137.

"I admire these guys so much, and when you can shoot your age in a major championship, they ought to give you bonus points," said Cook. "Lift, clean, and throw or something for the rest of the week."

Steve Pate (69) and Loren Roberts (67) share fourth place at three-under 139.

Cook started on the back nine Friday and birdied No. 10. He gave the stroke right back when he missed a two-footer for par at 11, but Cook steadied things with three straight pars.

He birdied the par-five 15th, but found trouble off the tee at 16, chipped out and made bogey. Cook birdied the 17th, parred 18 and made the turn at three- under for the tournament.

Cook tallied his first birdie of the second nine at three, then, after a par at four, caught fire. He birdied three in a row from the fifth, including a pair of five-foot birdie putts at five and seven.

Cook had chances at his last two, but made pars and that was good enough for a share of the lead.

"I'm very pleased with how I played today," said Cook. "I drove the ball nicely, I've kept my patience for two days, which is a record, so that's good."

Chapman began the second round on the 10th tee and collected his first birdie thanks to a 15-footer at the 12th. His only other birdie on his opening nine came at the 18th after he hit a wedge to three feet.

The Englishman continued his fine form at the first. He knocked a gap-wedge to 15 feet and poured in the birdie effort to reach six-under par for the championship.

Chapman laid up at the par-five eighth and the decision worked. He pitched his third to eight feet and rolled in the birdie putt. Two holes later, Chapman played an eight-iron to 25 feet and canned the putt to get to eight-under par.

He lost sole possession of the lead at the ninth. Chapman left himself 20 feet for birdie, but misread the putt and knocked it almost five feet by the hole. Chapman missed the par save coming back to fall into a tie for the lead at the midpoint.

"It left sort of a bitter taste after working so hard to get to where I was and just let one slip at the end there," admitted Chapman. "I maybe didn't quite play as well as yesterday, but certainly didn't make any real mistakes until the six at the last."

Joel Edwards (67) and David Frost (70) share sixth at two-under 140.

Michael Allen had quite a turnaround from Thursday to Friday. After a 77 in the opening round, Allen rebounded with a course-record 64 to jump into a tie for eighth at minus-one.

"It was sure a lot more fun than yesterday," said Allen, the 2009 Senior PGA Champion. "When you shoot that bad, you give yourself an opportunity to improve that much, you know."

Mark Calcavecchia, Jim Carter, Bernhard Langer, Lonnie Nielsen, Bob Tway, Boonchu Ruangkit and Peter Senior joined Allen six strokes off the lead.

NOTES: Cook owns eight Champions Tour victories, but no majors...Chapman has never won on the elder circuit, nor has he won on his home tour, the European Senior Tour...Jim Karr and J.C. Snead withdrew on Friday...The 36-hole cut fell at five-over par and Peter Jacobsen, Hal Sutton, Tom Kite, Fred Funk and last year's runner-up David Eger all missed the weekend.