Harrington captured last year's event with a 17-under 267 total.
He moved one shot clear of Scotland's Stephen Gallacher and two in front of fellow countryman Des Smyth and the Spanish duo of Miguel Angel Jiménez and Carlos Rodiles. European money leader Retief Goosen was in a group at 66.
Harrington's move to the top of the leaderboard was made possible thanks to a thrilling display around the turn, where he picked up five shots in five holes.
His run started with a eagle at the 540-yard seventh before hitting his tee shot to within five feet on the ninth to turn in 32. When he started the inward half with two more birdies he was two in front.
Harrington bogeyed the par-4 16th, his second bogey of the day, but he closed with two birdies. The score would have matched the course record of Eduardo Romero and Seve Ballesteros, set in 1991, had it not been for preferred lies which were in use for the first round.
Gallacher's 64 was his lowest round as a professional.