"It just wouldn't work anyway, because eventually they would still get you," Cachia told The World Game.
"If you sit 11 behind the ball and try to defend for 90 minutes you're probably doing your own blokes a disservice because you're basically handing over all the initiative to the opposition and saying 'we're just not good enough to compete at any sort of level'.
"Obviously we know Sydney are going to have the majority of possession and we know how well they attack, so we've got to try to do something to stop that as much as we can, but at the same time we've got to play."
Canberra are the only non-A-League left in the cup and have made it to the semi-finals without having to face an A-League club along the way.
But now their moment of truth has come against a Sky Blues outfit that is in great form starting their season with back-to-back 4-0 wins against Western Sydney and the Central Coast.
"If you didn't realise what you were up against before the A-League kicked off, they've underlined it twice already," Cachia said.
"So you approach it as best you can. You go out with a plan to combat that dominance, but at the same time you've got to stick to what you've done previously and which has been successful for you, because if you turn it all on its head you're probably not going to be doing yourself any favours."
Canberra will be missing midfielder Adam Rogic and winger Tom McLachlan from the team that beat Green Gully 1-0 in their quarter-final on September 27.
Rogic is suspended after being red-carded in that match and McLachlan is overseas on a trip that was arranged ahead of any suggestion that the team might still be playing this late in the year, when they would usually be deep into their off-season.
While Sydney FC are sure to be solely focused on the semi-final clash, Cachia plans to spend half of his day on Wednesday at his day job and anticipated most of his players would be working for at least part of the day or at least doing some studies, in the case of the younger ones.
He said there was never any thought of the team going into camp for the match.
"My guys are creatures of habit and to be honest we didn't even speak about going into camp," he said.
"We're playing at home and the guys are comfortable in their own surrounds and sleeping in their own beds."
Canberra have played a couple of practice games against local opposition in the last couple of weeks to keep in tune.
Cachia said he thought long and hard before delivering his message to the team ahead of the game.
"I've told the boys that, whatever happens - and I hate saying whatever happens because it almost sounds like you're admitting defeat - I just want them to be able to look back at this game and have no regrets," Cachia said.
"Just make sure they're as well prepared as they can be individually, they work as hard as they can in the game and do as much as they can and not walk away thinking 'if only I'd done more'.
"Remember, anything can happen in a football game. If you turn up thinking you'll lose, that's what'll happen, but as difficult as we know it's going to be we're going out there to try to win."