Goals from Barry Bannan and Tom Lees ensured Millwall suffered their first defeat of the 2018/19 season at Hillsborough on Wednesday night against Sheffield Wednesday in the Sky Bet Championship.
Ryan Tunnicliffe netted a late consolation for The Lions, but they lost 2-1.
There was one change to the starting XI as Tunnicliffe came in for Shaun Williams, whilst Fred Onyedinma took his place on the bench.
The first chance of the game came in the fifth minute and went the way of the hosts, but Steven Fletcher headed into the arms of Jordan Archer from just outside the six-yard box.
The Lions were to have their own chance at the other end a couple of minutes later but a Wednesday boot just prevented Steve Morison from stealing in at the back post to convert Shaun Hutchinson's header.
Tunnicliffe's booming tackle echoed around Hillsborough with the midfielder's through-ball sending Lee Gregory away, but Wednesday captain Lees was there to provide an inch-perfect challenge to stop the striker getting an attempt in on goal.
On 16 minutes Wednesday took the lead as Bannan's sweet strike from outside the penalty area crashed off the underside of the bar and left Archer no chance.
Five minutes later Archer was tested once more as Joey Pelupessy launched a shot towards goal from around 30 yards out, with the ball fortunately creeping just over the crossbar.
On 32 minutes The Lions had a six-on-three situation but the cross from Jed Wallace, with three Millwall players bursting into the box at the back post, was too deep and rolled out of play.
Bannan was proving a danger from distance and nearly scored a second 10 minutes before the half-time break, but a Millwall head was there to divert the ball behind. The resulting corner-kick saw another Lions breakaway but Wallace shot wide.
Gregory worked his way into the box on 39 minutes and twisted and turned, but was unable to get a shot away due to a strong Wednesday defence. Meanwhile, Jake Cooper headed wide in first-half stoppage-time.
Seconds from the whistle The Owls had the ball in the net after a delivery from Bannan was turned home, but referee Andrew Madley and his assistant judged the ball to have gone out of play shortly before the goal.
Mere seconds into the second-half it was two. A free-kick was swung deep into the box and defensive indecision allowed Lees to guide the ball home at the back post.
On 58 minutes Aiden O'Brien broke away down the right-hand side and squared for Morison, but his shot was blocked. Neil Harris introduced Tom Elliott and Jiri Skalak into the play on the hour-mark as The Lions looked for fresh ideas and the move did allow the visitors a period of dominance as they pinned the hosts back into the their own half, but to no avail.
With 72 minutes on the clock, Millwall finally registered on the scoresheet. A long ball forward from substitute Conor McLaughlin was chased down by Gregory and after the striker caught up with the play, he rolled a cross in for Tunnicliffe to fire home into the bottom corner.
A minute later Gregory snuck in at the front post and nearly directed a header home, whilst Skalak also had an attempt fall just wide of the post.
With The Lions on the top and the Hillsborough crowd nervy, Wallace then sent a dipping effort just over the crossbar with 15 minutes remaining.
The instrumental Bannan danced his way into the box in the 88th minute and smashed the ball across goal but substitute Atdhe Nuhiu could not convert. Skalak and Wallace put crosses into the box at the other end, meanwhile, but no Millwall man was there.
An almighty goalmouth scramble followed in injury-time but The Lions were unable to convert. Skalak also had a late attempt but it was saved by Cameron Dawson.
Sheffield Wednesday: Dawson, Palmer, Pelupessy, Fletcher (Nuhiu 69'), Bannan, Thorniley, Lees, Matias (Forestieri 70'), Reach, Preston (Boyd 79'), Penney. Subs not used: Wildsmith, Fox, Sam Hutchinson, Lee.
Scorer: Bannan 16', Lees 46'
Booking: Reach, Forestieri
Millwall: Archer, Meredith, Shaun Hutchinson, Cooper, J. Wallace, Gregory, Romeo, Tunnicliffe, Morison (Elliott 61'), O'Brien (Skalak 61'), Saville (McLaughlin 46'). Subs not used: Amos, Onyedinma, Ferguson, M. Wallace.
Scorer: Tunnicliffe 72'
Referee: Andrew Madley
Attendance: 21,349 (353)