Ranked 79th on the European Tour, Coetzee had six birdies and only one bogey.
Thomas Norret of Denmark, the only player to shoot a bogey-free round, was tied for second at 3 under with Matthew Zions of Australia, Eirik Tage Johansen of Norway, Andrea Perrino of Italy, and Benoit Teilleria of France.
Coetzee had a rocky start with his lone bogey coming on his fourth hole.
“You just have to keep plugging away and not try to hit it off the planet,” Coetzee said. “I thought to myself, ‘I can get really mad now or I can just keep trying to make pars.”’
Coetzee held his nerve and made a strong finish with birdies on his last three holes.
Zions also overcame a poor start to climb up the leaderboard. The Australian bogeyed two of his first three holes before making five birdies.
“It’s a good score considering the start I made,” Zions said. “I three-putted the first and third to be two over after three. But I think I only missed one green out there all day and I made some very good saves, which was nice as my putter has been pretty cold the last few weeks.”
Sam Hutsby of England was tied for seventh with six other players.
Defending champion Martin Wiegele of Austria was in 84th place after carding a 75.