Josh Coburn scored a brace against his former club as Millwall claimed a 2-1 Sky Bet Championship victory over promotion rivals Middlesbrough at the Riverside Stadium.
An energetic first half from the hosts saw skipper Dael Fry head them in front, but former Boro striker Coburn spearheaded the comeback for the Lions with two second-half goals.
Three points means Millwall now leapfrog Middlesbrough in the table, moving into the automatic promotion spots as the Teessiders drop into the play-off positions.
The hosts enjoyed a lively start when Callum Brittain's low pass whizzed through the area to Tommy Conway and his effort was deflected behind by Tristan Crama, with Alan Browne nodding over from the subsequent corner.
They came close again when a clearance reached Luke Ayling and the defender chipped the ball to Fry at close range but he volleyed over.
Casper De Norre had a long-range attempt collected by Sol Brynn, but Middlesbrough continued to dominate as Brittain's deep free-kick was headed over by Adilson Malanda.
Camiel Neghli had an effort blocked and a Boro break resulted in Anthony Patterson clearing Brittain's ball into the box.
Boro's bright start was rewarded in the 26th minute when Browne curled a cross in towards the back post for an unmarked Fry to head into the bottom corner.
Riley McGree threatened twice, having his first effort blocked and firing over from a second.
Mihailo Ivanovic's strike deflected narrowly past the post as the visitors searched for an equaliser and David Strelec was at the heart of another Boro break, having a shot turned behind.
The hosts were denied again when a corner was whipped in and the ball rattled around the box, with Fry's attempt brilliantly poked away by Femi Azeez just in front of the goal line.
Patterson was called into action early in the second half to hold Matt Targett's powerful strike from Strelec's pass and Millwall suddenly threatened when Brynn pushed away Crama's deflected effort.
A surge of pressure from the Lions resulted in an equaliser when Ivanovic hooked a corner into Coburn and his volley from the six-yard area hit Brynn, but the goalkeeper could not stop the ball from trickling over the line in the 58th minute.
A flurry of chances for both sides saw Brynn called into action to stop Azeez's menacing strike before holding Zak Sturge's header from a corner and at the opposite end, Patterson tipped Brittain's free-kick over the crossbar.
Conway launched into a mazy run from his own half but was denied by Billy Mitchell and Boro went close again when Aidan Morris' shot was blocked.
Coburn had the final say in the 86th minute after latching onto Barry Bannan's pass before dispatching a cool finish into the bottom corner.
Boro looked to snatch a late equaliser as successive crosses pinballed around the box at the death, but Millwall held on.
Middlesbrough's Kim Hellberg:
"Football is about scoring goals and we didn't do that good enough and win games.
"It's also the question more of what should you do - should you stop playing good football and stop creating chances? It's what can you do?
"When you concede two you're in big doubt of taking the three points, we didn't do it today.
"Of course it's concerning, we're working every day. It doesn't help to have all those players away that have been scoring a lot of goals for us.
"It's a concern, absolutely without a doubt. You have to fight and work through it. That's what we're trying to do. That's why the energy will be important.
"Tomorrow we go again and I'm very proud of the players, proud of the supporters. If there's someone to blame, it's me."
Millwall's Alex Neil:
"We'll see over the next six games how big a win it is but at the moment it feels like a huge win.
"They are a really good side. Really well coached. First half we couldn't really live with them.
"We had a chat at half-time. We needed to get the press a bit better. We needed to make sure we kept it in front of us a little bit more. When we did have it - that was our biggest issue. We kept giving the ball back to them.
"Second half we moved the ball better, the game slowed down in pace. Our goal then changed our belief.
"You couldn't have scripted it with Coburn coming back to his old side."
