Aaron Scott said scalping the Northern Premier League co-leaders was a positive pointer for the Hamilton hopefuls.
"The cup is always one game at a time. It's just one football field, 90 minutes and 11 men and anything can happen - look at the result we got today," Scott said.
"The team has got a good buzz and we don't start looking all the way. Who knows who we'll get in the semis - we'd love another home game but we may be way down south.
"The belief keeps growing. We've had three good wins against the top teams in our division and now against Birkenhead who are one of the better in the country but we outpassioned them today and once we got our noses ahead we had the belief to hang on."
Tomorrow another second tier side, Palmerston North Marist host Christchurch's Cashmere Technical in a 1pm kickoff while the two 2pm matches feature Waitakere City entertaining Onehunga Sports in Whenuapai and Olympic squaring off against Dunedin's Caversham in the capital.
The winners of this weekend's quarterfinals go into Monday's draw for the August 24 semi finals with the final scheduled for September 15 as the pinnacle of the winter football season.