PORT LOUIS, 8 May 2015 (AFP) - The 28-year-old two-time European Tour winner is one clear of compatriot Thomas Aiken and Denmark's halfway leader Thorbjorn Olesen at Heritage Golf Club.
Coetzee produced a stunning front nine, pitching in for eagle from the fairway on the seventh hole, following birdies on the second and fifth, before a further gain on the ninth.
He picked up another shot on the 14th hole to move to 11 under par for the tournament, with Aiken (68) and overnight leader Olesen (70) tied for second place on 10 under.
Coetzee said: "As I walked off the course, I told my manager I don't know how I'm shooting under par the way I'm playing, but obviously the course must suit my eye pretty well.
"Any way you get to a 65 is okay. I'm pretty happy but I made it difficult for myself on the back nine. But I felt like I hit enough good shots to maybe have some confidence for tomorrow."
Olesen, who led by one shot at the halfway stage, struggled with his driver but produced three birdies on the back nine, to remain in contention in his first tournament back after three months out with a hand injury.
"My par saves were pretty amazing today," Olesen said.
"I'm happy that we are done now because it was not pretty out there. I was trying to find an escape and get the best score I could. I hit a lot of poor tee shots.
"Hopefully I can find something overnight otherwise it is going to be a long round tomorrow."
Singapore's Mardan Mamat is a further two shots back on eight under par following a round of 67, with first-round co-leader Carlos Pigem of Spain among a four-man group a shot further back.
Spain's Javier Colomo, who on Friday became the first European or Asian Tour player to hit a hole in one on a par-four for an albatross, is 11 shots behind.
He carded a one-over score Saturday to be at par for the three rounds.
Leading third round scores in the European Tour/Asian Tour Mauritius Open at Heritage GC on Saturday (GBR/IRL unless stated, par 71):
202 - George Coetzee (RSA) 70-67-65
203 - Thorbjorn Olesen (DEN) 65-68-70, Thomas Aiken (RSA) 69-66-68
205 - Mardan Mamat (SIN) 69-69-67
206 - Carlos Pigem (ESP) 65-72-69, Dean Burmester (RSA) 67-68-71, Andrew McArthur 68-70-68, Nathan Holman (AUS) 67-73-66
207 - Jazz Janewattananond (THA) 69-70-68, Merrick Bremner (RSA) 71-70-66
208 - Pelle Edberg (SWE) 68-66-74, Matthew Fitzpatrick 68-67-73, Masahiro Kawamura (JPN) 68-69-71, John Parry 67-70-71, Oliver Bekker (RSA) 66-71-71, Tjaart Van der Walt (RSA) 73-66-69, Scott Hend (AUS) 72-66-70, Jake Roos (RSA) 70-71-67, Madalitso Muthiya (ZAM) 67-74-67