Duffy Waldorf, playing his 12th event on the Champions Tour, also posted a 66 in the first major tournament for senior players this year.
Haas, a 16-time winner on the Champions Tour, captured this event in both 2006 and 2008. He is looking for his first win on the circuit this year after having won at least once in six of the last seven seasons. Waldorf's best finish since turning 50 last year has been a tie for second.
Haas did not make a bogey at the Bellerive Golf Club, which in 1965 became what was at the time the longest course ever to host the U.S. Open.
Kiyoshi Murota and Sonny Skinner, one of 40 club professionals taking part in the nation's oldest senior tournament, shared third place at 67.