VERSAILLES, France, July 6, 2012 (AFP) - The 31-year-old German bounced back from a double-bogey six at the 17th, his eighth hole, with four birdies down the front nine, putting him into contention for a second European Tour win eight years after he took the Dunhill Championship in South Africa.
Siem leads at Le Golf National outside Versailles on six-under-par 136, one stroke clear of Sweden's Alexander Noren, who bogeyed the 18th, his last, but still had a 67 for the low round of the day.
Tied at four-under were Anders Hansen of Denmark, and George Coetzee of South Africa, both carding 68s.
Three were tied on 139 - England's David Lynn (72), Francesco Molinari of Italy (68) and top home hope Raphael Jacquelin (71).
Hansen had set the early pace, benefitting from the ideal early playing conditions.
The highlight of his round was three birdies in the tough four-hole closing stretch at the Albatross course, the host venue for the 2018 Ryder Cup. Most players would happily settle for level par for those.
Following a career-best third place finish at the USPGA last August, Hansen has had a torrid time of it this year with his swing unravelling mixed in with personal problems meaning that he has played a curtailed schedule.
But a change of coach appears to be paying dividends and Hansen feels that he can rebound over the rest of the year.
"This year has not been good at all," he said.
"I pulled out of a couple of tournaments due to just haven't felt great about the golf game and a few personal problems.
"But it's coming. It's feeling good now and I felt good coming into this week.
"I felt I made a really good decision about the team behind me."
Ryder Cup stalwarts Ian Poulter of England and Graeme McDowell of Northern Ireland were both well in contention at one under after rounds of 69.
England's Lee Westwood, the world No.3, had a 73 to slip out to one over after dropping five shots in four holes from the 12th in the worst of the afternoon weather.
Countryman Justin Rose also struggled to a 73 which left him at two over and there was more dismay for another Englishman, Paul Casey.
The injury-jinxed, former world No.4 had a second straight 80 to leave him holding up the field at 18 over par.
Also missing the cut was defending champion Thomas Levet of France who had a 76 to stand at six over for the tournament.
Leading scores on Friday in the second round of the French Open at the par-71, Le Golf National course:
136 - Marcel Siem (GER) 68-68
137 - Alexander Noren (SWE) 70-67
138 - Anders Hansen (DEN) 70-68, George Coetzee (RSA) 70-68
139 - David Lynn (ENG), 67-72, Francesco Molinari (ITA) 71-68, Rapahel Jacquelin (FRA) 68-71
140 - Brendan Steele (USA) 70-70, Ricardo Santos (POR) 71-69, David Drysdale (SCO) 70-70, Christian Nilsson (SWE) 65-75, Shane Lowry (IRL) 70-70, David Howell (ENG) 70-70
141 - Ian Poulter (ENG) 72-69, Soren Kjeldsen (DEN) 70-71, Jaco Van Zyl (RSA) 69-72, Shiv Kapur (IND) 72-69, Gary Boyd (ENG) 66-75, Matteo Manassero (ITA) 66-75, Branden Grace (RSA) 72-69, Hu Mu (CHN) 71-70, Graeme McDowell (NIR) 72-69, Henrik Stenson (SWE) 68-73, Thongchai Jaidee (THA) 66-75, Phillip Golding (ENG) 71-70