Thistle are going into the semi-final with Cowdenbeath on the back of a 3-1 defeat to Morton last weekend and the striker has vowed that the Jags are desperate to get that loss out of their system.
"It's a chance to win a medal and win a competition so we are all looking forward to it," he said.
"Hopefully it is a good day out for our fans as I am sure they would love to see us make it into a cup final.
"I think from the start that you always set out thinking that you are capable of winning it and the same goes for the league.
"You always have to believe and I think the way we have started this season highlights that.
"We have lost only once in the league and once in the League Cup but we have acquitted ourselves well. We want to keep our run going and basically try to win every game that we can.
"Getting into the final would be huge for us because it is a chance to win a bit of silverware and we're determined to go out on Sunday and give it our best shot."
Erskine has netted six goals so far this season and believes that he has already performed far better now than he did last term.
However, he insists that there is still more to come from him.
"I feel good," he said. "Last season I scored eight in the whole season and the target at the start of the campaign was always to beat that.
"I've not scored for a few weeks but I feel as though I have been playing okay and I still feel confident in front of goal.
"Being part of a winning team gives everybody a bit of confidence that you sometimes don't have when things aren't going well for you."
Having lost their league record to Morton, Erskine believes that a win against Cowdenbeath would get Jackie McNamara's side back on the right road.
He said: "We knew the run would come to an end but we've not been overly down about it.
"If someone had offered us this kind of start we would have bitten their hand off for it. As a team we are gelling well and performing well and we want to keep that going.
"The sooner we win a game the better because you always want to get back to winning ways once you lose a game.
"Cowdenbeath will fancy that they have a good chance against us but we have a few things to think about after beating them at Firhill earlier this season.
"But we'll be ready for it and the manager will have done his homework, that's for sure."