Ed Fiori and Gary McCord finish in a tie for fourth at 3-under-par 69.
Sigel, who captured the Bayer Advantage Celebrity Pro-Am this season, jumped out of the gate with a birdie at the opening hole at Hayfields Country Club. He faltered with a bogey at the par-3 third but recovered well with back-to-back birdies starting at the par-5 fifth.
The 59-year-old hit a brilliant approach to the par-4 ninth that skated by the cup and stopped within a foot of the hole. Sigel tapped in for birdie and made it two in a row with a 12-footer at the 10th.
At the par-4 12th, Sigel dropped his second shot inside four feet and converted the birdie putt. Sigel then ran home an 18-foot putt for a birdie at the par-4 15th to reach minus-6.
Sigel found trouble at the closing hole, however, and bogeyed the 18th for his share of first.
'I'm pleased with my score especially having not played here,' said Sigel. 'I hit it close a number of times today. All in all a solid round. I didn't mind the bad weather as long as it's not too cold.'
Nelson, who has not won on the Champions Tour since 2001, collected three birdies and an eagle over his first nine holes to make the turn at 5 under.
The 56-year-old landed his approach inside six feet for a birdie at the 13th but fell back into a tie for the lead with a bogey at the par-4 17th.
'Good solid round,' said Nelson. 'I've been hitting the ball well the last few weeks. Probably as good as I ever have.'
Smyth was rather quiet on the front nine but the Irishman began the inward half in style with three consecutive birdies starting at the par-4 10th.
He added a birdie at the 16th and hit his third shot to eight feet at the par-5 last for a closing birdie.
'This is a good start for me,' said Smyth. 'I'm really happy with my start. I'm usually a better finisher than a starter.'
Reigning Senior PGA Championship winner John Jacobs picked up three birdies and a bogey for a round of 70. He was joined by Mike Smith, Tom Purtzer, Tom Jenkins and Jim Holtgrieve.
Doug Tewell, Dave Barr, John Bland, Isao Aoki and Jerry McGee were one shot further back at 1-under-par 71.
Sam Torrance, the 2002 European Ryder Cup captain, made his Champions Tour debut and chipped in for a birdie at the 18th for a round of 74.
Defending champion J.C. Snead struggled with an opening-round 75.