Haas shot a 1-under 69 on the East Course of the Oak Hill Country Club, which has held the U.S. Open three times as well as the Ryder Cup. He won the Senior PGA two years ago in a playoff over Brad Bryant for his first and only major title.
Jeff Sluman, winner of the 1988 PGA Championship, shot a 70 to share second place with Bill Britton.
The group at 71 was led by Bernhard Langer, a two-time winner on the Champions Tour this year and a challenger at the recent Players Championship. He was tied for fourth with Scott Hoch, Joe Ozaki, Eduardo Romero, John Jacobs, Ian Woosnam, Keith Fergus, Kirk Hanefeld and Tim Simpson.
Haas stumbled near the end of his round with bogeys on the 16th and 18th.
"I played extremely well. I would have taken 1-under par before I teed off," Haas said. "But I'm still a little disappointed with the finish."