Bernhard Langer and Fred Funk were at 67 at Ocean Hammock Golf Club. David Edwards and Gene E. Jones were next at 68 in the Champions Tours deepest field this season.
Former Masters champion Craig Stadler was among those at 69 and defending champion Keith Fergus and Tom Watson were at 70.
Nielsen played on the PGA TOUR from 1978-83. His best finish was a tie for fifth in the 1979 Quad Cities Open. He worked as a club pro in New York State and qualified for the Champions Tour in 2003. Nielsen has won once, the 2007 Commerce Bank Championship.
He rallied from a double-bogey at the par-3 12th hole, his third hole of the day, and began a torrid run up the leaderboard with an 8-foot birdie putt at the 18th. Nielsen made the turn, then birdied Nos. 1, 2, 4, 6 and 7 to pull ahead of Langer and Funk.
Im feeling more comfortable out here, Nielsen said, referring to his progress on the tour. Thats half the battle. It sounds simple, but its a lot tougher than it sounds.
Langer hit the shot of the day, a 6-iron from 188 yards that he holed for an eagle-2 at the 13th. He also birdied three of the four par 5s. He made two clutch chips on his last two holes.
The long game wasnt quite as good as I wanted it, but the putter was good and the short game was good, Langer said. Ive had plenty of days where I played well and didnt get much out of it, so I would rather play not so well and score well.
Funk birdied the 16th to tie Nielsen for the lead, but dropped a shot with his only bogey of the day on the next hole. This matched his best opening round of the season in five starts on the Champions Tour and four on the PGA TOUR.
I played great, Funk said. The greens were real firm and real fast and it was a good test out there.