George Byers' 43rd-minute deflected strike ensured Millwall lost 1-0 to Swansea City in the Sky Bet Championship at the Liberty Stadium on Saturday afternoon.
In a game of few clear-cut chances, Byers scored at the second attempt shortly before half-time after his initial effort was blocked by the Millwall defence.
Millwall boss Neil Harris named an unchanged squad for the visit to South Wales, meaning that Ben Thompson, Ben Marshall and Jed Wallace all retained their places after returning in last weekend's draw with Rotherham United.
The Lions pressed high in the opening stages, but the first chance of the afternoon went the way of the home side as, with four minutes played, George Byers forced Jordan Archer into save from an effort just outside the penalty area.
Jed Wallace had Millwall's first effort of the game on 13 minutes, but after he ran on to Lee Gregory's clever header, his shot was blocked behind, with the corner-kick cleared away.
Ben Marshall's delivery into the penalty area from a free-kick was nodded across goal by Shaun Hutchinson on 25 minutes, but a Swansea boot was there to clear away from danger as a number of Lions players awaited. Two minutes later, Mike van der Hoorn could - and perhaps should - have given the home side the lead, but headed over, unmarked, from six yards out.
With 11 minutes to go until the half-time break, Swansea were presented with the chance of a free-kick from around 25 yards out - Matt Grimes struck well, but Archer was equal to his effort, tipping it behind.
With 43 minutes on the clock, The Swans took the lead. A corner-kick found its way to the back post, but after an initial shot was blocked, Byers' attempt spun into the top corner and beyond the helpless Archer.
The half-time substitution of Shane Ferguson for Murray Wallace nearly paid dividends just five minutes after the resumption - the Northern Irishman's cross eventually found its way to the back post and Mahlon Romeo, but the right-back's powerful shot was saved by Erwin Mulder.
As the clock struck 55 minutes, Millwall, looking more threatening in the second-half, nearly equalised through Jed Wallace, but the winger's drive was touched over by Mulder. The Lions were seeing a lot more of the ball and were still creating chances as the final 20 minutes approached, but they were unable to breach the home defence.
Substitute Oli McBurnie headed wide with 16 minutes remaining, whilst Millwall had Ferguson to thank a minute later as he raced back to clear Bersant Celina's casual finish off the line.
Grimes' free-kick on 88 minutes tested Archer, who palmed away, whilst a late effort from McBurnie trickled wide.
Deep into stoppage-time Jake Cooper could have equalised, but he turned his effort wide from close range after the ball fell at his feet.
Swansea City: Mulder, van der Hoorn, Celina, Routledge (Fulton 90+4'), Carter-Vickers, James, Grimes, Roberts, John (Naughton 60'), Byers, Baker-Richardson (McBurnie 70'). Subs not used: Nordfeldt, Dyer, Asoro, McKay.
Scorer: Byers 43'
Millwall: Archer, Hutchinson, Cooper, Williams (Morison 74'), J. Wallace, Thompson, Gregory, Romeo, M. Wallace (Ferguson 46'), Leonard, Marshall (O'Brien 84'). Subs not used: Martin, Onyedinma, Pearce, Tunnicliffe.
Booking: Marshall, Archer
Referee: Kevin Friend