New South Wales Waratahs must rediscover their killer instinct if they are to seize control of their finals fate in a high-stakes derby against the Brumbies on Friday night, Michael Cheika says.

Cheika was furious after the Waratahs almost frittered away a big early lead in their tense 18-16 win over Melbourne Rebels at ANZ Stadium on Saturday, saying the two-point escape was "far too close". Cheika also admonished his pack for losing their go-forward during "some very dark patches" in a pivotal encounter that almost certainly ended the gallant Rebels' own playoff hopes.

The hard-earned victory kept the Waratahs within striking distance of the Brumbies, four points adrift on the ladder, ahead of a bumper showdown in the national capital that will go a way towards deciding Australian conference honours, but Cheika said "I don't think anyone's going to be shaking in their boots down there".

Rob Horne scored the critical try on Saturday © Getty Images

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The Brumbies dished up the Highlanders in Canberra © Getty Images

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Still, a first victory over the Brumbies in Canberra since 2011 will vault the Waratahs to the top of the conference with a game in hand over their arch rivals, and Adam Ashley-Cooper said: "That's the goal of ours, to be leading our Australian conference and we want to continue the momentum that we got over the last two weeks.

"Having that first semi at home is huge. I mean, finishing top guarantees you a spot in the finals, and that's exactly where we want to be. So we can control our own destiny by getting the result." NSW won the round-six meeting in Sydney 28-13 in a heated affair headlined by homophobic slurs from Jacques Potgieter being fined, and Michael Hooper expects another physical contest.

"We always have a big confrontation against these guys," said Hooper, whose breakdown battle with Pocock will be a feature - and potentially decisive.

Ashley-Cooper said neither team would be lacking motivation. "We haven't won there for the last couple of years. The last two times we've travelled down there, we've been absolutely dished up.

"We probably didn't give the Rebels the respect that they deserve because, in the end, they could have got a penalty and could have stolen the game for us. We've got to learn from that. We've got to become a little more ruthless when things are going our way, rather than sitting on our heels and expecting things to happen."