His Moors side dominated throughout against United at Brewery Field but had midfielder Keith Graydon to thank as his goal from a free-kick on 73 minutes was all that separated the sides at full-time.
As the final whistle approached and with his side still unable to break the deadlock, Ainsley said doubts did start to creep into his mind.
Spennymoor Town 1-0 Causeway United
The FA Carlsberg Vase
Fourth Round Proper
Saturday 18 January
The Brewery Field
Attendance: 644
He said: "In games like this you pray for an early goal. They came and parked the bus and had a good game plan, but it was one of them where we were just camped in their half with our goalkeeper not having a save to make.
"The longer it went on you start to worry - 'It's not going to be our day' - and we missed chance after chance but I think it was fitting the way it ended.
"Keith has had a terrible month after losing his father recently, and I felt that moment of magic from him was well deserved and I couldn't be more delighted for him."
It is often said success is built on victories such as Spennymoor's on Saturday, and despite only converting one of their numerous chances, Ainsley was happy to get the job done and move one step closer to Wembley.
“Anyone who plays Spennymoor will know they will be in for a tough game”
Jason Ainsley
Ainsley said: "For five minutes after the goal I think we got a little bit excited, and they put us under a little bit of pressure, but our objective at three o'clock was to be in the next round.
Ainsley will find out who his side face in The Fifth Round on Monday and he is naturally hoping for another home tie.
He added: "Anyone who plays Spennymoor will know they will be in for a tough game and likewise we know that there are great challenges ahead.
"We are all happy to be in the last 16 as we want to retain our trophy and have another experience like we had last May."
