Just like the spring sun, Lady Luck shone on Fleetwood to help them secure a historic first-ever League One play-off spot – and that is just the minimum this team will now achieve with two games to go.
Ben Davies picked the perfect time to score his first career goal seconds before the half-time whistle to cut the between Town and second-placed Bolton to one point.
The pressure is all on the Wanderers after Southend’s shock 2-1 home defeat to MK Dons to guaranteed Fleetwood a top-five finish.
Rosler made two changes to the side that beat Peterborough 2-1 on Good Friday, with Wes Burns and Cameron Brannagan coming in for Ash Hunter and Kyle Dempsey, who moved to the bench.
But Town immediately had their backs against the wall after a nervous start that saw Millwall do everything but score in a blistering 25-minute spell.
Millwall should have taken the lead in the fifth minute as Ben Thompson’s long-range effort beat keeper Alex Cairns but crashed into the left post, with Lee Gregory firing the rebound straight at the Town shot-stopper.
The Lions roared again moments later, with Steve Morison blasting the ball at Cairns from an angle and Byron Webster also firing straight down the keeper’s throat as Millwall bombarded Town.
Fleetwood had a rare attempt on goal as Bobby Grant blasted over after neat work by George Glendon and David Ball on the left.
Mllwall went close again as Shaun Hutchinson nodded a set-piece just over.
The visitors threatened again as Shaun Williams’ corner from the right fell to the feet of skipper Morison at the back post but his effort was tipped away by Cairns.
Neil Harris’ men continued to dominate as Jed Wallace stormed down the left flank before teeing-up Gregory in space in the box. His strike clattered into the right post and moments later O’Brien blasted over – and all before the 25-minute mark.
But Fleetwood managed to win more possessio and play, with Grant forcing King into an acrobatic diving save from a free-kick.
Fleetwood’s confidence grew and the Lions were nearly the architects of their own downfall as Conor McLaughlin’s cross in from the right was back-headed towards his own goal by Hutchinson under no pressure. Webster got in ahead of Burns to clear off the line.
Grant curled a slick effort from a short corner over the bar and Davies nodded a Glendon corner wide for resurgent Fleetwood.
Another well-worked set-piece routine saw McLaughlin fizz an effort just past the left post after Glendon and Brannagan linked up.
And it was from a standard set-piece routine that Fleetwood finally broke the deadlock. Mahlon Romeo had needlessly fouled Bell on the left flank and Glendon whipped the free-kick into the mix for Davies to head home his first Town goal.
Fleetwood started the stronger in the second half as Burns wriggled towards the back line and forced King to tip his angled effort over, though Millwall continued to push forward to keep their play-off push intact, Gregory forcing Davies into a last-ditch block on the line.
Bell was too ill to last the full game on his 100th Town outing and the full-back made way for Victor Nirennold as McLaughlin switched flanks.
Rosler took Burns off and pushed Bobby Grant up front but it was Millwall who should have scored next only for man of the match Cairns to pull off an incredible one-handed save to stop Cooper’s goal-bound header.
The Lions went to four up front for the final 15 minutes as they chased the game, with O’Brien and later Cooper joining Gregory and Morison as they tried to force an equaliser.
But some fierce defending from Nathan Pond and a last- gasp Cairns save from Calum Butcher ensured the Lions were tamed as Town clinched another huge three points.