Millwall manager Neil Harris hailed his team's shock 1-0 win at Derby as one of the best away displays of his managerial reign.
A resilient defensive performance was capped by a Jed Wallace breakaway goal which clinched only a second away win in 18 league games.
Millwall set out to frustrate Derby and carried out the plan to the letter as the play-off hopefuls could not find a way through the ranks of orange shirts.
The first half was devoid of excitement or incident and Derby did not improve in the second, although Jack Marriott had a glimpse of goal in the 50th minute only to lose his balance as he shaped to shoot.
Jed Wallace skimmed the Derby crossbar with a cross five minutes later and he landed the decisive goal in the 72nd minute as the Lions scored with a fast breakaway.
Shane Ferguson sent Wallace running into the left side of the penalty area and his low drive under Kelle Roos was the game's first shot on target.
Derby, who gave a full debut to Ashley Cole, did not test Jordan Archer until the second minute of stoppage time when he parried a Fikayo Tomori header, and Harris said: "We knew we were coming up against a very talented Derby side who have an excellent home record.
"So we knew we would have to be disciplined and organised because of the ability they have to open you up.
"First and foremost I want to praise the team's defensive ability because I think the 93rd minute was their first effort on goal. It's certainly one of the best away performances of my time in charge.
"The best praise I can give my players is that was a very, very impressive Millwall performance.
"Any clean sheet at the level we play at is a huge bonus. We built our foundation last year on it and we've tried in the second half of the season to get that mentality back again."
Derby manager Frank Lampard admitted: "There were no surprises, Millwall are very organised and we had a load of the ball and lots of possession.
"But we weren't quick enough with it at times and we didn't have enough imagination in our game to break them down and they scored with their only shot and only attack.
"So it was a culmination of us not being bright enough on the ball and then having a lapse that gave them a goal, but it's a game where if you're not at your best you should draw 0-0.
"I wouldn't be surprised at anything in this league but I thought we would cause them more problems, but we didn't create enough."
