Hahn, who gained European Tour status via last year's European Tour School, produced eight birdies and an eagle in his round of 61 to lead by two over Portugal's Ricardo Santos.
England's Oliver Fisher is one shot further back in 3rd position with David Horsey another shot back again and alone in 4th.
The pre tournament favourite, Thomas Aiken is in a share of 5h five shots from Hahn's lead.
Wade Ormsby did best of the two Australians in the field, the South Australian in a share of 19th position at 8 under.
Rookie West Australian, Brady Watt, missed the cut.