They didn't falter, with du Preez scoring his second try of the match near the posts.
The Brumbies missed the composure of rested five-eighth Matt Toomua, with the backline not looking as crisp as it has in recent weeks.
Despite dominating both possession and field position, the Brumbies handed over the ball throughout the match with handling areas, aimless kicking and poor passing despite a blistering 20-minute opening performance.
The Brumbies flew out of the blocks, with an early penalty to Christian Lealiifano and tries to unmarked wingers Henry Speight and Joe Tomane.
The Kings hit back after the restart in the 22nd minute, stringing together about a dozen phases before loosehead prop Schalk Ferreira burrowed over next to the posts.
Kings flanker Wimpie van der Walt then crossed the line from the back of a rolling maul in the 33rd minute after five-eighth George Whitehead put a penalty kick on the halfway line five metres out from the Brumbies line.
The visitors were the first to score again after the break when the Brumbies lost possession five metres out and the Kings went the length of the field to put du Preez under the posts for his first try.
Ignoring booing from the crowd, the Brumbies took three points whenever on offer throughout the match to keep the scoreboard ticking over, with Lealiifano kicking five from five penalty attempts and Nic White one from three.
Brumbies captain Ben Mowen said he felt the TMO decision after the siren was correct despite it costing the Australian conference-leading Brumbies a win against the Kings, who sit last in the South African conference.
"Absolutely fair enough, I think it's a valuable part of the game to have a TMO that can come back and check those things,'' Mowen said.
"So the call is right, Benny did play the bloke without the ball there is no doubt in that.''
Coach Jake White said he felt the two missed conversions early in the game allowed the Kings to feel as though they were still in the match.
"That's Super Rugby, as soon as you give them a sniff, they score a try and make it 13-7 and grow another leg,'' he said.
Kings director of coaching Alan Solomons said he thought his side had lost the game as he was unaware a TMO decision was taking place.
"We came down from the box, I thought the game was gone. We stopped to look at the TV and I thought it was a replay,'' he said.
"Then I look again and say `no, this isn't a replay, we can still draw'.''
Solomons said the Kings showed grit and character to come back despite being a man down when prop Ferreira was sent of in the 65th minute.
"That was a bit of a worry actually when he went off. That put us under a lot of stress,'' he said.