Millwall were defeated 3-2 by Watford in a behind-closed-doors friendly at The Hornets' training ground on Wednesday afternoon.
Domingos Quina gave the Premier League outfit a first-half lead, but two goals in quick succession from Matt Smith and Murray Wallace turned the game on its head after the break. Merely a minute later, however, Adam Masina levelled the scores, before Ashley Fletcher scored a late winner.
Gary Rowett named two different XI's for the fixture, and the first-half side began strongly, with Danny McNamara and George Saville having shots on goal early on. A Jed Wallace free-kick was then nodded just wide by Daniel Ballard, before the first Watford shot came on 14 minutes with Tom Cleverley striking over.
The Lions were moving the ball well against their higher opponents and nearly fashioned the opening goal on 18 minutes, but after Jed Wallace broke through the defence and fed Mason Bennett, his effort was stopped by Ben Foster.
Jed Wallace and Cleverley traded blows again as the half entered its last 10 minutes, but with 36 minutes on the clock, Quina's free-kick from just outside the penalty area evaded the dive of George Long to open the scoring.
Watford's new XI started the second-half brighter, with Bartosz Bialkowski producing an outstanding save to tip a Fletcher effort over the crossbar, and that proved to be pivotal as The Lions raced up the other end in an attack which ended with Smith rising highest to power home Connor Mahoney's free-kick.
Barely 120 seconds had passed when Millwall went into the lead, as a corner bounced around the edge of the penalty area, allowing Murray Wallace to loop a header over the despairing dive of the goalkeeper and hand The Lions a second.
However, just a minute after that, Masina was able to equalise, stooping to head home on the edge of the six-yard box.
The Lions were undeterred by the equaliser, with some fantastic, free-flowing and attacking football ending with Mahoney threading Ben Thompson through on goal, but he flashed his attempt wide, before a driving run from Tyler Burey was blocked by the 'keeper and Mahoney cracked the crossbar from 25 yards out with 11 minutes to play, but the winner was to go Watford's way as Fletcher scored with four minutes to play.
The fixture acts as one of a few behind-closed-doors friendlies at training grounds over the next couple of weeks ahead of the 2021/22 Sky Bet Championship campaign.
Millwall First-Half XI: Long; McNamara, Ballard, Hutchinson, M. Wallace, Malone; Kieftenbeld, B. Mitchell, Saville; J. Wallace, Bennett.
Millwall Second-Half XI: Bialkowski; Romeo, A. Mitchell, Pearce, Cooper, M. Wallace (Burey); Evans, Thompson, Mahoney; Smith, Afobe.
Scorers: Smith 57', M. Wallace 59'