Irish striker Andy Keogh scored twice as Millwall clinched their first home win of the new Championship season against Middlesbrough at the New Den.
Liam Trotter put the Lions ahead on 25 minutes with a thumping 20-yard volley after James Henry's flick.
Henry struck the Boro post before Keogh coolly converted the rebound for Millwall to double their lead.
The visitors pulled a goal back through Marvin Emnes's 42nd-minute penalty but Keogh's late strike sealed victory.
Middlesbrough, who started the day in the top six, will have not appreciated the lunchtime kick-off in south-east London, having not arrived home from their midweek League Cup victory at Gillingham until 4am on Wednesday.
And they started sluggishly after another lengthy journey back down to the capital as Millwall raced into a two-goal lead in the opening 35 minutes.
Midfielder Trotter fired into the top corner past Boro keeper Jason Steele, before Keogh, who had been a constant menace, reacted quickest to slot home.
Middlesbrough were given a lifeline three minutes before the break when Jimmy Abdou fouled George Friend in the penalty area and Emnes calmly struck home from 12 yards to reduce the deficit.
But any hopes of a comeback evaporated when Keogh tapped in Scott Malone's cross from the left.