Morris Hatalsky and Tom Jenkins are tied for fourth place after rounds of 5-under-par 67.
Irwin began his round on the front nine of the Founders Course at Greystone Golf & Country Club. He began slowly with just two birdies on the front nine. He rolled in a 15-foot birdie putt at the par-4 third, then another one from 10 feet out at the par-4 sixth.
Around the turn, Irwin got hot. He birdied the 11th from eight feet to move to minus-3. Irwin, who has 36 Champions Tour victories, then carded back-to-back birdies from the 13th.
The 2001 Bruno's winner closed with a long birdie at No. 16 and another one at the par-5 last from seven feet.
'I've had some success here in the past,' said Irwin. 'My intention tomorrow is to hit it in the fairway on the first hole and continue working on the good things I did today.'
Fleisher, who nearly withdrew because he is battling the flu, got started with three straight birdies starting at the par-4 sixth. Around the turn, Fleisher birdied the par-4 12th from eight feet out to get to minus-4.
The 54-year-old, who won the Verizon Classic earlier this year, followed that birdie with an eagle at the par-5 13th. Fleisher dropped a shot at the 17th when his five-footer for par slid by the edge but he was able to share second place thanks to a birdie on the par-5 closing hole.
'I got up this morning and made plans not to play,' said Fleisher. 'The flu started two days ago and I was real close to not playing. I hit a lot of wonderful iron shots today. When you don't feel well it takes a lot of the anxiety away.'
Ziegler had an up-and-down start. He birdied the second, but dropped a shot at the next. The two-time winner on the Champions Tour then rolled off three consecutive birdies from the fifth to make the turn at minus-3.
On the back side, Ziegler drained a five-footer on the 11th for birdie. He made two straight birdies from the 14th to get to 6-under. He faltered with a bogey on 16, but erased that mistake with a birdie on the 17th.
'I started using a belly putter because my son Tony was bugging me to try it,' said Ziegler, who is playing on a sponsor's exemption. 'Its pretty nice when you make some putts. The belly putter makes it so much easier to roll the ball.'
Bob Gilder, Tom Wargo, Graham Marsh and Des Smyth are tied for sixth place at 4-under-par 68.
Defending champion Sammy Rachels is tied for 45th place at even-par 72.