Nick Flanagan posted Saturday's lowest round with a 6-under 66. That vaulted him into a tie for second place with Adam Crawford, who shot a 71, at minus-5.
Phil Tataurangi (68), Paul Sheehan (69), Michael Long (73), Jarrod Lyle (73) and Bradley Iles (73) are knotted in fourth place at 4-under-par 212.
Windy conditions and fast greens made for a brutal Saturday as only seven players broke 70. The course played to an average of 73.066 and several golfers were higher on the leaderboard until coming to Royal Adelaide's difficult closing holes.
Sim played well out of the gate on Saturday with birdies at the second and third holes. He dropped a shot at the 160-yard, par-3 seventh, but made the turn under par.
Like most other players in round three, Sim struggled on the back nine. He bogeyed the 13th, but seemed to minimize the damage with back-to-back birdies from the 14th, including a birdie putt from the fringe at 15.
Trouble loomed for the man who turned professional in November of 2005. Sim bogeyed the par-3 16th as well dropping a shot at No. 17, the hardest hole through three rounds.
'Those finishing few holes can really get you if you're not on top of your game,' admitted Sim.
He hung on to par 18 for the 54-hole lead in search of his first win on the Nationwide Tour.
'I am a little surprised to be leading the tournament after an even-par round today,' said Sim. 'Everybody was moving up and down today. I'm just enjoying the moment.'
Flanagan's round featured two eagles - a 5-footer at two and a 10-footer at nine. He mixed four birdies and two bogeys the rest of the way for his impressive 66.
'I hit the ball really well today,' said Flanagan. 'I putted better today after not holing much of anything the first couple of days. I'm still trying to win the tournament.'
Crawford's round featured four birdies and three bogeys.
Kevin Stadler, who won last week's Johnnie Walker Classic on the European Tour, shot an even-par 72 and is tied for ninth place with Scott Laycock (68), Marc Leishman (69), Parker McLachlin (70), Peter Senior (72), Aron Price (72) and Michael Wright (72). The group came in at 3-under-par 213.
Overnight co-leaders Ryan Armour and Spencer Levin both struggled to rounds of 5-over-par 77 and are part of a group tied for 16th place at minus-2.