Jay Haas, 45, is closely followed by Tiger Woods, 23, Hale Irwin, 54, Sergio Garcia, 19, among others.
A game for all ages, golf knows no generation gap. It allows an AARP member to compete alongside a teenager, with various levels of youth and experience represented between them. Forget about birth certificates at the PGA Championship.
Jay Haas was the unlikely leader through 36 holes at Medinah Country Club, a 45-year-old veteran of 23 years on the PGA Tour whose best days are behind him.
Yet he was far from the oldest player on the leaderboard.
Hale Irwin, 54, who has been all but driving a Brinks truck full of money away from Senior PGA Tour events, was just four shots behind, bidding to become the oldest winner in the history of major championships.
Already the oldest winner of the U.S. Open, having captured that tournament on this same Medinah No. 3 course nine years ago, Irwin played the first two rounds with a player who was not born when he finished third at the 1975 U.S. Open here.
Sergio Garcia, 19, was tied with Irwin, four shots back. Between them and the lead was Mike Weir, 29, Tiger Woods, 23, and Lee Westwood, 26.
"You just put a bag over everybody's head, so you can't look in the mirror and see a gray hair and you can't look in the mirror and see a kid with freckles," Irwin said. "You don't pay attention to how old they are. They are players in the field who are trying to beat you. If you feel as if you're going to be beaten by these players, you will be beaten. I don't like to think that way.
"I look at it as the ultimate challenge when you come out and play on a golf course like Medinah in a major championship against the best players in the world. In my sport, what better situation can you be in?"
Haas knows the feeling. He has competed in dozens of majors and even played as an amateur in the 1975 U.S. Open at Medinah, where he was paired with Jack Nicklaus during the third round and made an impressive showing, tying for 18th.
He turned pro in 1977 and won nine times, although he has no victories since 1993.
"It would mean a great deal, because I've played a long time out here without a major win," said Haas, who shot 67 to complete 36 holes at 135, 9 under par. "We all want to win golf tournaments and majors, especially, and I've not done that. I'm disappointed that hasn't happened. I'm not devastated by it, but we all think we can do better, no matter what. Golfers are like that. And any championship is a thrill."
Chasing Haas was Weir, a left-handed Canadian who finished second to Woods last month at the Western Open. He shot 68_136. Woods was another shot back after 67 that included birdies at the first three holes, followed by Westwood who shot 68. Irwin was one more back, tied with Garcia, Stewart Cink and Skip Kendall, who shot 65, the competitive course record.
Medinah has not been the monster most expected. Rain on Thursday significantly softened the greens, and thunderstorm doused the course late Friday.
That made scoring considerably easier, although a number of name players missed the cut, which came at 146, 2 over par. Among them were Lee Janzen, Ernie Els, Justin Leonard, Greg Norman and Tom Watson.
Irwin, who has won 21 senior events over the past three seasons, acted as if people shouldn't be surprise that he is still here and some others are headed out of town.
One of golf's most enduring memories is of Irwin dancing around the 18th green at Medinah after holing a 60-foot birdie putt on the 72nd hole, putting him in a playoff that he won for his third Open title.
Now he's a senior tour star and often jokes that he belongs there. It didn't look like it Friday, when he added 3-under-par 69 to his opening 70. A lot of players would love to be where Irwin is now.
"It's golf. It's not a marathon," Irwin said. "I'm not trying to get into a strength test or a stamina test with them. I'm just trying to play golf. I still have a few skills that are there. I'm not throwing in the towel quite yet."
"It surprised me the way he hits the ball," said Garcia, who turned pro in April and won the Irish Open last month. "I knew he was a great player, but I didn't know he was that good. He hits the ball very straight, he rarely misses a fairway and he's a good putter."
Garcia would be the youngest winner of the a major championship this century. Irwin the oldest ever.
Which is more impressive?
"Both," Garcia said.