Adcock mixed seven birdies with two bogeys -- including one at 18 -- for his first overnight lead ever on the Champions Tour. Ben Crenshaw and Jay Sigel share second place at 4-under 68, one shot ahead of five players tied for fourth.
Defending champion Des Smyth, Tom Purtzer, Bruce Fleisher, Gil Morgan and John Bland are all at minus-3. Smyth opened with consecutive bogeys before rallying for his 3-under 69.
Adcock was also an open qualifier at the Turtle Bay Championship earlier this year, finishing tied for 67th in Hawaii. A veteran of the PGA Tour -- and one- time member of both the Nationwide and European Tours -- he tied for 43rd at Champions Tour Q-school last fall.
Recent lessons with golf instructor Jim Hardy have given Adcock confidence in his game.
'He's made all the difference,' said Adcock, 50. 'I paid $2,000 to go to his seminar and I knew after I hit the tenth ball [that] it was right.'
Adcock played with Bill Longmuir and Pat McGowan in the final group off the first tee. He made the turn at 4-under-par after four birdies on the front nine -- including one on a 30-foot putt at the 421-yard, par-4 fourth.
Two more birdies at 11 and 12 were spoiled by his first bogey at the par-3 14th, where Adcock missed a 5-foot par putt. He went on to birdie the par-5 15th and bogey the par-5 18th.
'This tour wasn't built for guys like me,' Adcock added, before joking about the weekend: 'An 18-hole tournament suits me. I hope some weather comes through.
'We're just trying to see how far we can take it.'
Almost as weird as Adcock's surprising lead was the round of Lanny Wadkins, who made five birdies, one eagle and seven pars, but ended at just minus-1. A double bogey and four bogeys has him tied for 15th place.
Loren Roberts, winner of three events already this season, is also at 1 under.