Haas finished 36 holes at 11-under 131, good for a three-stroke lead at Glen Oaks Country Club in Iowa.
Trailing him at minus-8 was Larry Mize, who hung onto second place with a 3-under 68.
One shot further back was Tom Lehman, who registered a 4-under 67.
Champions Tour rookie Andrew Magee matched his Friday total with a 3-under 68, good for fourth place at minus-6.
Mike Goodes fell from his leader's perch with a 3-over 74, dropping him into a share of ninth at minus-4.