Millwall 2 Swans 0
Swansea City tasted defeat for the first time under manager Roberto Martinez on a disappointing day at The Den where Alan Tate was sent-off after just 22 minutes.
Tate's second yellow card so early on gave the Swans a mountain to climb and they slipped four points away from the last play-off place occupied by Blackpool who beat Crewe 2-1.
With the Swans down to 10 men, Millwall took full advantage and secured the three points courtesy of a Paul Robinson opener and a Neil Harris penalty.
Millwall: Pidgeley, Senda, Robinson, Shaw, Brammer, Hackett (Elliott 57), Dunne (Ardley 79), Smith, Byfield (Hubertz 2), Harris, Craig. Subs not used: Day, Ashton.
Swansea City: Gueret, Tate, Iriekpen, Painter, Butler, Britton (Darryl Duffy 72), O'Leary, Robinson, Craney (Richard Duffy 46), Abbott (Trundle 46), Lawrence.
Subs Not Used: Williams, Oakes.
Att: 9,249
Ref: R Lewis (Shropshire).
Swans boss Roberto Martinez made just one change from the side that beat Chesterfield and Northampton at the Liberty Stadium.
Kristian O'Leary was recalled the middle of midfield with Alan Tate dropping to right-back in place of Kevin Amankwaah.
The Swansea manager also resisted the opportunity to bring Lee Trundle straight back from a two-match ban, with the striker starting on the substitutes bench.
Lions boss Donachie made two changes to the side beaten at Port Vale, with on-loan midfielder Ryan Smith coming in for his full first-team debut from Derby and Chris Hackett returning to the starting line-up.
After a tight opening period, Millwall were given the psychological edge when Tate was booked for fouls on Paul Herbetz and then Ryan Smith.
It forced Swansea into another reshuffle as Leon Britton dropped back into the right-back spot.
But Britton struggled to cope with the outstanding Smith who set up Millwall's opening after 31 minutes.
His cross struck the top of Swansea's bar and when the ball dropped to the feet of Hubertz , Paul Robinson was perfectly placed to slot home his pass from two yards for his first goal at The Den in two years.
Harris and Danny Senda came close for the home side soon after, while Willy Gueret did well to deny Harris just before the break.
Swansea made two changes at the start of the second-half with Trundle and Richard Duffy coming on for Pawel Abbott and Ian Craney.
But Swansea were dead and buried in the 69th minute when Tommy Craig played Harris in and the striker was brought down by Izzy Iriekpen inside the area.
Referee Rob Lewis had no hesitation in pointing to the spot and Harris picked himself up to slot the penalty beyond Gueret.
Swansea did try and respond through efforts from Andy Robinson and Tom Butler, but the best chances still fell to the home side with Harris on the end of most of them.
Swansea's first defeat in six games increases the importance of the Easter campaign with a trip to Bristol City on Saturday followed by a home game against Port Vale on Monday.
With six games remaining, Swansea can ill-afford many more slip ups in their quest for a play-off place
