Jerry Pate, still winless on the Champions Tour, is alone in second place after a 5-under-par 67. Bruce Fleisher bogeyed his last, but is in third at minus-4.
Dana Quigley, the tour's leading money winner, is 345 points ahead of Hale Irwin, a four-time winner on the Champions Tour this season. Eleven other players have a chance to win the title as well, but the race seems to be between Quigley and Irwin.
Advantage Quigley after the first round. He three-putted the final hole on Thursday, but still managed a 3-under-par 69 and is part of a group in fourth place.
Irwin struggled badly on the front nine with three bogeys, but made up the lost strokes with a 12-foot birdie putt at the 15th and an eagle at the 16th. He is at even-par 72 and is tied for 20th place.
They will all be looking up at Nielsen after round one.
Nielsen, 28th on the Champions Tour money list, broke out quickly with a 10-foot birdie putt at the first. He made it two in a row at the par-5 second when his pitching-wedge approach stopped 18 feet from the hole.
The 52-year-old looked to be in trouble at the par-4 sixth, but holed a clutch 4-footer to save par. Nielsen closed his front nine with a 5-foot birdie putt at the ninth to make the turn at 3-under-par 33.
Nielsen collected his second birdie in a row at the 10th when his sand-wedge landed 3 feet from the cup. He parred the next two holes and made a nice birdie at the par-5 13th. Nielsen found the bunker with his second, but blasted out to 5 feet and converted the birdie try.
At the par-4 15th, Nielsen drained a 15-footer to save par and stay bogey-free. He hit his second to 10 feet on the par-5 16th, but missed badly. He did roll in the 4-footer for birdie and sole possession of first place.
'I'm thrilled. The way I warmed up on the range today, that was quite a surprise,' admitted Nielsen, who is winless on the Champions Tour. 'The putter was steady, and I drove the ball in the fairway most of the time. That's the key out here, to stay out of the rough.'
After the first round, Nielsen acknowledged that playing with a fellow former club professional, Mark Johnson, helped him ascend up the leaderboard on Thursday.
'It was a real comfortable pairing for both of us,' said Nielsen. 'There is a bond there for us club pros, but to get a chance to come out here to play against these big named players we have been watching our whole careers, it's great.'
Quigley was joined in fourth place by Tradition winner Loren Roberts, Mark James, Tom Watson, Craig Stadler, Des Smyth, Gil Morgan, Tom Jenkins and D.A. Weibring.
Irwin was matched in 20th by defending champion Mark McNulty.