Bourdy had five birdies in his second nine and finished with a flourish in building a two-stroke lead over Fredrik Andersson Hed and Oliver Fisher.
Andersson Hed had four birdies in his final four holes in his 6-under 65 while Fisher overcame an early bogey en route to his share of second.
Five players are tied for fourth at 5-under 66. They include Anirban Lahiri, who was bogey-free Thursday, and Rhys Davies, who started his round with an eagle.
Bourdy opened the tournament on the second nine at the Crans-sur-Sierre course and posted a birdie at the 10th hole. He added birdies at 15 and 17 and made the turn at 3-under.
He jumped up the leader board with birdies at Nos. 1 and 2 and took the outright lead with three birdies over the four-hole span from 5-8.