JOHANNESBURG, 4 Dec 2014 (AFP) - Fisher, 34, who had the last of his five European Tour wins at the Tshwane Open in South Africa in March, carded a six under par 66.
Two shots back of the Englishman at the Gary Player Country Club in Sun City are South Africa's George Coetzee, Siem of Germany and French player Alexander Levy.
Siem produced perhaps the most remarkable shot of the day.
With an eight iron, his 183 yard approach to the par four 17th deflected off the right edge of the green and spun into the hole for an eagle that won the German a Volvo car.
Siem, who explained he will give the car to his mum, said: "It's not the best golf shot at all for me in my career. I tried to draw it in and it stayed straight and hit the fringe and had a firm bounce to the left. It was really cool to hole that one."
Fisher dropped just one shot in his round, at the par three 16th, and made all of his gains in birdies.
Having picked up shots at all four par fives on the course, he said: "I think the par fives are all scoreable. They are all reachable, but you've still got to hit two good shots.
"So par fives are quite key, but again, there are more scoreable holes out there. There's quite a lot of short par fours where if you get your driver in good position, you've got short irons into the greens, and the greens are putting well."
First round scores in the Nedbank Golf Challenge at the Gary Player Country Club, South Africa on Thursday (GBR/IRL unless stated; par 72):
66 - Ross Fisher
68 - Marcel Siem (GER), George Coetzee (RSA), Alexander Levy (FRA)
70 - Brooks Koepka (USA), Charl Schwartzel (RSA), Stephen Gallacher, Miguel Angel Jimenez (ESP), Louis Oosthuizen (RSA), Tim Clark (RSA)
71 - Luke Donald, Danny Willett, Thongchai Jaidee (THA), Jonas Blixt (SWE)
72 - Kiradech Aphibarnrat (THA), Joost Luiten (NED), Shane Lowry, Lee Westwood
73 - Dawie Van Der Walt (RSA), Brendon Todd (USA), Pablo Larrazabal (ESP)
74 - Tommy Fleetwood, Jaco Ahlers (RSA), Jamie Donaldson, Martin Kaymer (GER)
75 - Marc Warren
76 - Mikko Ilonen (FIN)
77 - Kevin Na (USA), Danie Van Tonder (RSA)
80 - Thomas Bjorn (DEN)