After shooting a stunning first round nine-under par 62, Fox followed it up with a solid four-under effort in his second round, leading by a shot over American Connor Arendell.
Fox had an inconsistent start to the round, with three birdies and two bogies peppering his front nine, before picking up the pace with three back-nine birdies to post a 67 and remain atop the leaderboard at the French tournament.
His strong start follows his excellent 49th placed finish at the Open Championship last weekend.
The Challenge Tour is a second-tier circuit - the European Tour's equivalent of the Web.com Tour, and Fox sits in 46th on the money list despite only having played three events so far this season.
On the PGA Tour, Kiwis Steven Alker and Tim Wilkinson have both made the cut at the RBC Canadian Open.
Wilkinson shot a two-under par 70 to make the cut on the number at two-under, while an eagle halfway through his round saw Alker card a three-under 69 to move to four-under for the tournament, in a tie for 44th.
Fellow New Zealander James Beale - an amateur who has played on the Canadian Tour, shot consecutive rounds of 75 to miss the cut.
American Chad Campbell leads at 14-under after a scorching second-round 63.
