Josh Coburn's header earned promotion-chasing Millwall a fourth straight win in the Sky Bet Championship as they ground out a 1-0 victory against Derby at The Den.
The Lions rode their luck during a nervy end in which the Rams passed up a succession of chances as their play-off push took a blow.
But for 24 hours at least, Millwall - who have won seven of their last nine games - find themselves one point behind second-placed Middlesbrough, who face Charlton on Wednesday night.
Derby almost struck against the run of play after 15 minutes when Patrick Agyemang's header from Joe Ward's corner was cleared off the line by Femi Azeez.
While Millwall had the better of the opening stages territorially, it took them until the halfway mark of the first half to have their first real attempt as Mihailo Ivanovic's free-kick was deflected wide.
But the chances started to come the Lions way as Azeez's cross picked out Zak Sturge, whose header was pushed away by Derby goalkeeper Josh Vickers.
From the resulting corner, Coburn beat Vickers to Camiel Neghli's ball into the box and his header came back off a post.
The breakthrough for the hosts arrived in the 43rd minute when Barry Bannan's free-kick was headed up by a Derby defender, with Tristan Crama doing well to hook the ball back in for Coburn to head in from close range.
Coburn almost had a second two minutes later when he cut inside from the left before his low effort from the edge of the area was pushed behind by Vickers.
Finding themselves behind at the break, Derby needed to find more of a spark in attack and captain Lewis Travis dragged a shot wide from outside the area with the hour mark approaching.
The Rams had a huge chance to equalise after 69 minutes when Agyemang rolled away from Crama to move in on goal, only to drag his shot horribly wide.
The visitors started to crank up the pressure following this missed opportunity, however, and started to pin Millwall back in their own half for the first time in the game.
Sam Szmodics was the latest of their players to threaten a leveller when he advanced into the area before his shot across goal travelled wide.
Another big Derby chance went begging in the 90th minute when Bobby Clark's cross was nodded back by Agyemang for Carlton Morris, who could not find the target with his diving header.
There was one final scare for the hosts in the fourth minute of stoppage time when Agyemang's shot across Anthony Patterson hit a post, meaning there was much relief at the final whistle.
Millwall's Alex Neil:
"You could see the difficulty that we couldn't keep the ball at the top end of the pitch because when we took Josh and Femi [Azeez] off, Matt Clarke was heading the ball 60 yards up the pitch every single time.
"It meant we were defending our box for the last five minutes, but the earlier I did that I knew that was going to be a problem, and I tried to delay it but I also need to protect some of my players.
"I knew with every player I was putting on we were getting smaller and smaller, but we've done what we needed to do.
"For 60 minutes, I thought we were comfortably the better side.
"We deserved it over the piece, we rode out luck at the end - they missed one or two opportunities which we probably should have defended better, but the game lost its shape completely."
Derby's John Eustace:
"I'm obviously very disappointed with losing the game because in the second half I thought we were excellent.
"We have to be better in both boxes. First half I thought we dealt with a lot of pressure, but we probably weren't competitive enough in midfield.
"We made a good change at half-time which certainly allowed us to be more competitive and more on the front foot, and we're disappointed not to have at least come away with a point.
"We've played Hull now, who are fifth, and we've played Millwall, who are third, and we've gone toe-to-toe with two very good teams.
"I felt like we could have got more out of both of those games.
"You can talk about performances as much as you like, but it's about winning, and we haven't won the two games, but I think you can see the progression we've made throughout the season."
