Wanderers succumbed to a narrow 2-1 defeat to Millwall after extra-time in tonight's Carling Cup clash at Adams Park, with Alan Dunne sending the Championship side into round two after Ben Strevens had cancelled out Neil Harris' opener.
Harris opened the scoring midway through the first half when he followed up with a rebound after Danny Schofield's effort was parried by Nikki Bull, ending a spell of dominance from the visitors.
But the Blues were vastly improved after the break and earned their equaliser, which came in some style when Matt Phillips produced a masterclass in wingplay to dash past a host of Millwall defenders and give Strevens the simple task of tapping home from six yards.
Wycombe finished the game strongest but couldn't find a winner, and they were left to rue a couple of missed chances when Dunne won the game for the visitors in extra-time with a close range finish from John Marquis' lay-off.
With five changes to the side that kicked off the league campaign with a 2-0 win over Morecambe at the weekend, Gary Waddock's men were forced onto the back foot for much of the opening half of this cup clash, with the visitors showing the kind of form which earned them promotion to the Championship just three months ago.
And in fact by the time they took the lead on 25 minutes, they could have been at least or three goals ahead. Nikki Bull made a smart save to deny Jack Smith's rasping drive and Kevin Lisbie missed three good chances to open the scoring before Neil Harris tapped home when Danny Schofield's initial shot was parried into his path.
The goal prompted a response from the home side and they began to compete with the Lions, with the pace of Matt Phillips in particular looking capable of creating openings.
The winger saw a free-kick flash over the crossbar on 35 minutes before Kevin Betsy forced a save from David Forde moments later after some skilful play inside the visitors' box.
But despite the late spell of pressure, Gary Waddock would have had every right to be disappointed with his side at the half-time interval, as they had misplaced countless passes and allowed too much space for the visitors to work in during the first period.
So it was no surprise to see the Blues show plenty of improvement at the start of the second half, with long-range efforts from Kevin Betsy and Danny Foster showing their intentions early on.
The introductions of debutant Andy Sandell and Matt Bloomfield around the hour-mark helped inject fresh impetus into the home side and the Lions seemed happy to sit on their one-goal lead rather than push themselves for a second, allowing Wycombe to enjoy plenty of possession in the final third.
However, goalscoring chances were hard to come by for the Blues and Gary Waddock swapped Ben Strevens for Scott Rendell in the target man role in a bid to swing the game into Wycombe's favour. And it worked, all thanks to the majestic work of Matt Phillips on the wing.
The England Under-19 international danced through a host of yellow shirts after bursting goalwards from the touchline and laid the ball on a plate for Strevens to tap home his first goal for the club from six yards.
Millwall were stung into action and forced two smart saves from Nikki Bull in quick succession, with the keeper reacting quickly to deny Neil Harris and Danny Schofield at his near post.
The game was opening up. A strong burst by Jon-Paul Pittman created a chance for Lewis Montrose to fire goalwards, with David Forde making the save, and Wycombe finished the 90 minutes as the side looking more likely to find a winner.
As is often the case, the start of the extra-time period was cagey, with both sides starting to show signs of fatigue and a willingness not to fall behind at a time when chances are often hard to come by.
Nikki Bull was twice called into action, saving from substitutes Alan Dunne and John Marquis in quick succession, but he could do nothing to deny Dunne on 105 minutes when he slid home after Marquis' lay-off inside the area.
Wycombe cranked up the pressure after the break and searched for a goal to take it into extra-time, but couldn't find many clear-cut chances and in the end had to be grateful for Alan Bennett's goal-line block to prevent the visitors stretching the lead.
Millwall managed to hold out and secure their passage to the second round of the competition, but despite dominating for the first half hour, they left Adams Park knowing they'd been given one hell of a challenge from Gary Waddock's men.
Wycombe: Bull, McCoy, Foster (Sandell 59), Bennett, Johnson, Phillips, Montrose, Murtagh (Bloomfield 62), Betsy (c), Rendell (Strevens 68), Pittman.
Subs: Arnold (GK), Westwood, Westlake, McClure.
Goal: Strevens (76)
Millwall: Forde, Smith, Barron, Robinson (c), Mkandawire, Ward, Henry (Hackett), Laird (Dunne 68), Schofield, Lisbie (Marquis 99), Harris.
Subs: Mildenhall (GK), Frampton, Grabban, Grimes.
Goal: Harris (25), Dunne (105)
9mins: CHANCE - Centre-back Darren Ward flashes a shot wide after the ball fell to him on the edge of the 18-yard box.
13mins: CHANCE - Nikki Bull dives to his right and fists away Jack Smith's rasping drive from 20 yards.
14mins: CHANCE - Kevin Lisbie shanks his effort wide from only eight yards out.
17mins: CHANCE - Bull is called into action once again, this time to palm away Lisbie's curling effort.
19mins: CHANCE - Lisbie hits Bull's near post with a low drive after latching onto Ward's ball over the top.
25mins: Scott Barron fires straight the throat of Bull from the left apex of the Blues' 18-yard box.
25mins: GOAL - Neil Harris fires home from close range after Bull saves well from Barron.
35mins: CHANCE - Matt Phillips curls a free-kick just over the top of David Forde's goal from 20 yards out.
38mins: CHANCE - Kevin Betsy forces Forde into a smart save from just inside the area after turning away from Paul Robinson.
42mins: CHANCE - After some smart build-up play, Marvin McCoy's teasing cross is fisted away by Forde.
48mins: CHANCE - Kevin Betsy's low drive flashes past David Forde's right hand post.
50mins: SAVE - Danny Foster's 30-yard effort is well held by the visiting keeper.
55mins: CHANCE - James Henry's free-kick sails harmlessly over the top.
59mins: SUB - Andy Sandell comes on for his Wycombe debut, replacing Danny Foster at left-back.
62mins: SUB - Matt Bloomfield replaces Keiran Murtagh.
68mins: SUBS - Ben Strevens comes on for Scott Rendell while Millwall introduce Alan Dunne in place of Marc Laird.
76mins: GOAL - Superb work from Matt Phillips provides Ben Strevens with the simple task of tapping home from six yards.
78mins: SAVE - Nikki Bull does well to deny Neil Harris and Danny Schofield in quick succession.
FT: Wycombe Wanderers 1-1 Millwall
94mins: CHANCE - A fierce low drive from distance from Matt Phillips just misses the target.
99mins: SUB - Kevin Lisbie comes off for John Marquis.
100mins: SAVE - Alan Dunne's header is well held by Nikki Bull.
105mins: GOAL - Dunne makes no mistake this time, sliding home from John Marquis' cutback.
114mins: CHANCE - Two more efforts from Dunne, one of which flies over the bar and the other is blocked on the line by Alan Bennett.
FT: Wycombe Wanderers 1-2 Millwall (AET)