The three-time European Tour winner carded nine birdies and two bogeys at Pula Golf Club where he won the 2007 Mallorca Classic.
"I always like to come back to a tournament [venue] that I won," Bourdy said. "I have very good memories, a good feeling on it. I couldn't have a better start than this, so it's great. I'll try to do the same tomorrow."
His score placed him one shot ahead of Danny Willett, who packaged seven birdies and a bogey into a 64.
Darren Clarke, who has 12 European Tour wins, and Matthew Nixon, who has yet to win on the tour, are tied for third at 65. Rhys Davies held fifth place at minus-four.
Scott Jamieson, Chris Wood, Cesar Monasterio and Jaco Van Zyl are knotted in sixth at 3-under 67.