The winger turned in a man-of-the-match performance to see off Raith Rovers at Firhill, taking two goals with aplomb and setting up the other for strike partner Steven Craig in a convincing 3-0 win.
The Thistle faithful have taken Erskine to their hearts as he has excelled this season, with the talismanic forward again proving his worth to the Jags as Raith were swept aside in an eight-minute showcase at the start of the second half.
McNamara said: "It was a crucial spell for us. We were clinical with our chances.
"Erskine scored both with his right foot. His first; his touch was great. The ball has come across to him and he has opened it up and slotted it. The second one was a good move as well which Steven got on the end of. I am delighted for him.
"The first goal was going to be crucial and Scott Fox had a great save from their corner. We knew they would be dangerous at set-pieces, that was their biggest danger against us last season.
"Foxy pulled off that save at 0-0 and once we got that first goal, Erskine had a 12-minute period where he was outstanding and almost unplayable. I am delighted with the lads. We have got some really good players here and we need to keep showing that on a regular basis."
Jags fans have become accustomed to seeing plenty of goals on their trips to Firhill this season but they had to a while wait for the floodgates to open yesterday afternoon.
They had first-half chances, Paul Paton and Sean Welsh firing wide from close range and Stephen O'Donnell seeing his shot saved by Rovers keeper Allan Fleming, but Thistle were not at their crisp, effervescent best in the final third.
That was to change however, as they soon turned on the style, with Erskine at the heart of the Jags' attacking forays. His first of the afternoon owed much to the industry of Craig, the loan star bursting down the left wing and finding his forward partner unmarked, but his own skill proved vital too as he controlled and fired low into Fleming's net.
Just minutes later, the roles were reversed as Erskine's cross enabled Craig to stab the ball home from close range.
The best was still to come from the Jags' mercurial striker as he put the tie beyond Rovers' reach before the hour mark, collecting Paton's raking pass with a sublime touch before showing his composure to beat his man and curl the ball beyond Fleming from the edge of the area to clinch Thistle's place in the last four.
McNamara said: "I think we can play better. There wasn't much in the first half; it was a bit scrappy. The first goal was crucial in the game, we got a lift from that and got the shackles off and started passing. There was a 12-minute period when we killed the game.
"There were times in the first half when we were pulled down. Credit to Raith, they are well organised and tried to stop us passing it through them.
"I said to the lads that they could only do that for so long, it is difficult to do it for 90 minutes. I told them to keep plugging away and doing things early and we would get the rewards, which we did."