Pappas moved clear of Fiji's Vijay Singh, John Huston and PatPerez in second place, while Rocco Mediate, David Peoples and Dan Forsman shared fifth on 65.
And while Casey has reached the last two finals of the World Match Play in Arizona, Harrington has lost in the first round to PatPerez and Jeev Milkha Singh.
Anthony Kim lies sixth on eight-under with PatPerez a further shot back and Brian Gay on six-under with India's Jyoti Randhawa -- the only other non-American inside the top eight.