Derby County 0 Millwall 3
Millwall maintained their impressive run with a comprehensive victory at Pride Park that highlighted Derby's pressing need for a central defender.
Derby were exposed by crosses into their penalty area, with Barry Hayles exploiting their weaknesses with a hat-trick.
It was Millwall's fourth win in five games and the only blot on an otherwise satisfing afternoon for the Londoners was the sight of player-manager Dennis Wise being taken off on a stretcher. Wise suffered a back injury 10 minutes from the end, but it was not believed to be serious.
Certainly Wise would have been encouraged by his team's performance against another of the Championship's form sides.
Derby had won at Wigan on Sunday, but Millwall had beaten another of the promotion's favourites Ipswich and they repeated that display for a thoroughly deserved victory.
Hayles got the headlines for his three goals, but Wise and Jody Morris were excellent in midfield and ran the show after Derby had started brightly.
Andy Marshall made several important saves, but Millwall were always in control after Daniele Dichio headed a free-kick back across goal and Hayles headed into the top right corner.
It needed an excellent save from Lee Camp to deny Morris a second in the 32nd minute, but Millwall were not to be denied and Hayles pounced again five minutes into the second half. A good pull-back from David Livermore allowed Hayles to get in front of Michael Johnson and squeeze a low shot inside Lee Camp's left-hand post.
Derby responded with a double substitution but the game was over when Hayles completed his hat-trick in the 57th minute. Once again it was a cross from the Millwall left but that caused the damage with Hayles easily heading home from six yards past the unprotected Camp.
From then on Millwall played out the game in comfort, although Marshall made another smart save three minutes from time to deny substitute Paul Peschisolido.
It was Derby's first home defeat since November, but they need to strengthen their defence if they are to have any chance of challenging for the play-offs.