Bruce Fleisher fired the low round of the day Saturday with an eight-under 64 to tie Dana Quigley (70) for fourth place at eight-under par.
Vicente Fernandez, Mike McCullough, Bob Gilder and Monday qualifier Morris Hatalsky shared sixth place at minus-seven.
Sigel held a two-shot lead after Friday's opening round and dropped a stroke with a three-putt bogey at No. 5. He wasted no time making up for the miscue, however, birdieing the sixth and eighth holes to make the turn at one-under 35.
Sigel missed the green at the par-four 11th and couldn't recover, taking a bogey on the hole. But he was once again quick to reclaim the lost stroke when he two-putted the par-five 12th for birdie and ran home a four-footer for birdie at the 13th to get to minus-10.
He parred his final five holes to take the 36-hole lead -- always a good position, but especially in this tournament. In the previous eight years of this event the player who has led or been tied for the lead after 36 holes has gone on to win seven times.
'It's nice to be in the lead but it doesn't mean a whole lot,' said Sigel, the winner of the Farmers Charity Classic three weeks ago. 'I have hit a lot of good shots and I'm pleased with the score I shot. I've got a job to do and I'll try to go out and do that.'
Zoeller had an up-and-down round Saturday. He collected three birdies and four bogeys on his front nine but soon settled in, birdieing three of the first four holes on the inward half at Springhouse Golf Club. Zoeller closed with a six-foot birdie at 17 and a three-footer for birdie at the last for a 68.
'I don't really care if I'm in the lead or not,' said Zoeller, whose breakthrough win at last week's major marked his first trip to the winner's circle since 1986. 'I'm just glad that I'm close.'
Morgan tallied five birdies and one bogey for a round of four-under 68.
Fleisher, who received an exemption to play in this week's U.S. Open as a result of winning last year's U.S. Senior Open, wound up declining the invitation, saying he didn't feel he could compete at Bethpage's Black Course, the longest layout in the history of the championship.
Fleisher heard criticism over his decision from fellow seniors Hale Irwin and Tom Kite, both of whom went to Bethpage and missed the cut.
'I did what I wanted to do with the U.S. Open,' said Fleisher. 'I always felt an obligation to be part of the Senior Tour. I think there's a time and place and you have to move on.'
Miller Barber bettered his age by four Friday with an opening-round 67. He failed to follow up the success on Saturday, posting a 10-over 82 for a share of 69th at five-over par.