Last season, Adelaide City beat Western Sydney Wanderers 1-0 in the round of 32. Last night. Edgeworth Eagles almost pulled off a shock result against Melbourne City, losing 2-1 to a goal deep into injury time after leading 1-0.
Harnwell, who had a tremendous playing career with Perth Glory in the A-League and the old NSL before that, likes the sound of such results.
"That sort of result, particularly when you see Adelaide City beat Wanderers, gives you confidence," Harnwell said.
"And if you look at the FA Cup in England, there's always one result every year that is a big upset and which people look back on. It's up to us to make sure we're the club that does that in the FFA Cup this year and be remembered as the team that knocked off Sydney FC."
It is a similar scenario to the Adelaide City-Wanderers game for Sorrento - a team playing at NPL level against the previous season's beaten A-League grand finalist.
But what does Sorrento, which is currently running fourth in the NPL WA, have to do to somehow manufacture a win against the hot favourites in tonight's clash?
"Obviously you've got to go into every game believing you can win," Harnwell began.
"Do I believe we can play them off the park and dominate possession 80 per cent? No, of course not, but I believe we can get the team up to get a result and that's what we've been working on for four or five weeks.
"Everyone's got to do their job to the best of their ability. We're a strong side, a physical side - we've always had that - but I've also been getting the boys to play a lot more football than we have done in the past.
"We've been fairly successful at that, we can get the ball down and play, but they (Sydney) have got a lot of young kids in their team so we want to make sure we're physically aggressive as well.
"It's a tough ask, a big ask, but we're a proud club and we've battled against the odds before. Probably never this big, but we'll have a good crack at it.
"We don't want to sit back and defend for 90 minutes and allow Sydney to dictate terms, so the two things we've been working on are keeping possession of the ball and making Sydney work harder as a result, and keeping the concentration levels up.
"The difference between A-League players and others is often that they can concentrate for 90 minutes, whereas you might flick a switch on for 10 minutes and then switch off and suddenly - if you're playing against an Alex Brosque or a Shane Smeltz - it could cost you three goals in a hurry."
Lack of suitable floodlighting meant the game was switched from Sorrento's home ground to nib Stadium, when Perth Glory plays, and that made Sydney coach Graham Arnold happy because of the high-quality playing surface.
But Harnwell said his team could also knock the ball around a bit and that he was happy to play there as well.
"We expect to get something going," Harnwell said. "We've got a plan of how we want to play. We're not going to bomb forward with six players up front or anything like that, but we've certainly got our ideas on how we think we can break them down, how we can hurt them.
"Set pieces are going to be important as well. If you can pinch a goal from a corner or a free kick, that sets the opposition back on their heels and vice versa. A great goal against us from a situation like that would make it difficult to come back into the game.
"So there are a lot of different aspects where it could be won or lost. If our players play at 100 per cent, we've got a chance of winning. If we don't, then it's going to be difficult for us."
Harnwell said that just as his players were looking forward to taking on the Sydney stars, he was keen to duel with Arnold.
"Obviously Graham Arnold has been there and done that, he's a fantastic coach," Harnwell said. "I get to try to match wits against him and try to unlock what he does.
"We've worked very hard, with extra sessions, to try to fine-tune what we want to do and hopefully the plans we put in place work. If they don't and I've got it wrong at the end of the day, if you're the coach you put your hand up.
"Sydney has got plenty of experience and that's the test for us - can we find a way to match the A-League pros in a one-off situation?"
