Joel Grant gave Wycombe a sensational 13th-second lead and Dean Morgan added a brace as Wanderers upset the odds to seal a well-deserved victory at League 1 Portsmouth.
Morgan made it 2-0 before Izale McLeod reduced the deficit, however, Morgan struck again after seeing his initial spot-kick saved to seal a memorable night in Hampshire.
Chairboys were better than their opponents in every department and march on to the Southern area quarter-finals.
Nikki Bull returned to the Wycombe side and QPR loanee Bruno Andrade was in for his Wanderers debut.
Morgan fired over a minute later as Portsmouth failed to get going and Andrade also looked lively early on, causing the experienced Jon Harley all sorts of problems. Josh Scowen was the latest Wycombe player to try his luck, but Eastwood was in the perfect place.
Ainsworth’s side got off to a dream start and took the lead after only 13 seconds. Jo Kuffour collected the ball in the Portsmouth half and found Grant on the left-hand edge of the area. He cut inside and curled a sublime right-footed effort past Simon Eastwood.
Wanderers’ start almost got even better in the eighth minute when he waltzed through the defence but saw his curling striker bounce back off the post when he should have scored.
The hosts responded well with Bull having to be at his best to deny first Izale McLeod and then centre-back Gabor Gyepes.
However, that second goal came less than 60 seconds later when Kuffour was twice denied and Morgan made no mistake from close-range.
Johnny Ertl then tested Bull again, but his strike was straight at Blues’ No.1 and it was the visitors that came inches away from scoring again.
The lively Kuffour did superbly well to turn away from one defender and he then curled past Eastwood from fully 20 yards, but again the ball bounced back off the post. It was a fine effort from the striker and he deserved a goal.
Portsmouth started to see more of the ball, but they struggled to create clear chances and Buzsaky’s 30-yarder, which was saved by Bull, was the closest they came in the opening 44 minutes.
However, Pompey grabbed a lifeline in first-half injury-time and it came in controversial circumstances. Charles Dunne looked to have won a throw-in, but the assistant awarded it to the hosts and from that attack McLeod finished from close-range after Wanderers failed to clear their lines.
Wes Thomas and Gabor Gyepes both had shots early on in the second half, but neither troubled Bull.
And it was the visitors who struck again in the 54th minute. Andrade played a one-two with Kuffour and was upended in the area by Adam Webster. Morgan stepped up from the spot, but his penalty was superbly saved low to his right by Eastwood, however, the forward reacted quickest and headed in the rebound.
The hosts upped the pressure, but Wycombe were holding firm with the whole side defending superbly, led by the magnificent Dave Winfield.
Portsmouth created very little as they chased the game and Morgan had a late goal disallowed for a push on a defender as the Chairboys comfortably ran down the clock to claim a well-deserved and famous victory.
Wycombe: Bull, Foster, Dunne, Winfield, Stewart, Andrade, Scowen, Spring, Grant, Morgan, Kuffour (Morias 80).
Subs: Ingram, Logan, Kewley-Graham, Doherty, Morias.
Portsmouth: Eastwood; Webster, Harley, Michalik (Harris 60), Gyepes, Williamson, Ertl, Howard (c), Buzsaky (Rodgers 78), Thomas, McLeod
Subs: Walker, Compton, Obita, Harris, Rodgers