Watson matched his all-time low mark that he originally set in the third round of the 1999 Bank One Championship. The 62 equaled the lowest 18-hole score in a major championship and tied the tournament 18-hole mark set by Doug Tewell in 2001.
The previous 36-hole tournament record was 132 and that was first established in 1991 by Phil Rodgers and later equaled by Gil Morgan in 1997. The four-shot margin at the halfway point is the largest since 1991 when Rodgers held a six- shot edge.
'I'm on a good roll now, but I'll need to stay aggressive,' said Watson, who is bogey-free in two rounds at The Reserve Vineyards and Golf Club. 'I didn't have a five on my card. That's a rarity.'
Ahern posted a 4-under 68 on Friday while Irwin, who is still battling back problems that plagued him at the U.S. Open in June, birdied his last five holes en route to a round of 5-under-par 67. The duo is at minus-10.
Fuzzy Zoeller, the other co-leader after round one, carded a 2-under 70 and shares fourth place with Tom Kite and Morris Hatalsky, who shot matching rounds of 4-under 68 on Friday. The trio stands at 8-under-par 136.
Watson wasted little time in breaking into red figures on Friday. He holed a pair of six-foot birdie putts at one and three, then birdied three in a row from the fifth, including two 20-footers and a tap-in at the par-5 sixth.
That run prompted some amazing thoughts in Watson's head.
'I was thinking about 59 after the seventh hole,' admitted Watson. 'I choked. I had a few sloppy shots in the middle of the round.'
Watson missed the green at the 10th, but pitched to a foot to set up the kick-in par. He also missed the putting surface at the par-3 11th, but got up and down for par after converting a six-footer.
Watson got back on track at the 12th. He knocked a sand wedge to three feet for birdie before another birdie tear. Watson sank a 35-footer for birdie at the 14th, two-putted from 50 feet at the par-5 15th and rolled home a three-footer at the par-5 16th.
The reigning Senior British Open champion parred No. 17, but took advantage of the par-5 18th, the third five-par in the final four holes. He laid up and wedged his third to five feet. Watson stroked home the birdie putt to take a big lead into the weekend.
'Lots of things went right,' understated Watson. 'I got a lot of good breaks. I made a few sizeable putts as well. The conditions made it easy, but it's one hole at a time.'
Ahern held the early clubhouse lead and got there with strong play on the second nine. He collected a pair of birdies at five and seven, but opened his back nine with a six-foot birdie putt at the 10th.
He dropped a shot at 12 when his ball plugged into a bunker but after back-to- back two-putt birdies at 15 and 16, Ahern is in the hunt for his first major title in close to four years on the elder circuit.
'I played pretty good,' said Ahern. 'I didn't make many long putts, but I did make four putts of seven feet or less that kept my round going. That was the difference between 68 and 72.'
Irwin, who has never won this major title, plodded along at even-par on Friday until he hit the par-3 14th hole. He sank a 10-footer for birdie there and holed a birdie putt from similar length at the next hole.
Irwin two-putted for his third birdie in a row at the par-5 16th, birdied No. 17 and drained a 30-footer for birdie at the last to cap off his second consecutive round of 67.
'I struggled to keep any semblance of scoring going,' said Irwin. 'I made an awful lot of long putts. I feel like I'm going out there with half the clubs in my bag.'
Defending champion Jim Thorpe posted a 4-under 68 on Friday and shares seventh place with Graham Marsh, who fashioned a round of 70. That duo stands at 7-under-par 137.
Bruce Fleisher (70), Mike McCullough (69), Dave Barr (69), Vicente Fernandez (68) and D.A. Weibring (68) are tied for ninth place at minus-6.
Jack Nicklaus, a four-time winner of this event, fired a 5-under 67 on Friday to vault into a tie for 14th place at minus-5.