|Gregory 23, pen 36, pen 50||Venue: The Den
|Ince 42, Martin 70 pen, Hendrick 85|
LEE Gregory's first professional hat-trick was not enough for Millwall to beat Derby, who fought back from 3-1 down to draw at The Den.
Gregory capitalised on Lee Grant's error to put the Lions in front and then doubled the lead from the penalty spot after Lee Martin's shot was handled.
But Tom Ince reduced the deficit on the stroke of half time when his soft free-kick floated over a poorly-built wall and crept in at the near post.
Gregory completed his treble and gave Millwall breathing space again in the 50th minute, converting from 12 yards following a shove on Alan Dunne.
The Lions' two-goal cushion was halved again, however, when Ince went to ground under Aiden O'Brien's challenge and Chris Martin scored from the spot.
O'Brien and Martyn Woolford were both denied by the fingertips of Grant during a spell of dominance for the home side, but the Rams carved out a fine late leveller.
A deep cross into the box was nodded down by Ince into the path of Jeff Hendrick, who struck a brilliant first-time volley across David Forde and into the top corner.
Within seconds of Hendrick's goal, Rotherham equalised against Norwich and can now relegate Millwall by beating Reading on Tuesday.
For the Lions to stay up, they must hope their rivals fail to win in midweek and again on the final day, while they would have to defeat Kenny Jackett's Wolves.
Neil Harris made three changes to the side that were beaten 2-0 by Blackburn, with an injury to Mark Beevers prompting a recall for Jos Hooiveld at centre back.
Martin and Gregory also came into the Millwall XI at the expense of Magaye Gueye and Ricardo Fuller, who both dropped to the bench.
Before kick off, Sid Nelson was named the club's Young Player of the Year for 2014-15 and Jimmy Abdou collected the senior award from chairman John Berylson.
After a scrappy opening ten minutes, Martin won and took Millwall's first corner of the afternoon and his delivery took a nick off a defender before flashing dangerously across goal.
In the 16th minute, O'Brien led a Millwall counter-attack with both Abdou and Gregory up in support, but the youngster went for goal and Grant held his low drive with ease.
The Rams threatened for the first time when Ryan Shotton's long throw into the box was glanced just wide by Richard Keogh, with Forde struggling to get there.
Dan Harding sliced an awkward half-volley well over before Nicky Bailey was tackled as he went to pull the trigger after playing a neat one-two with O'Brien.
But when Grant gifted Gregory possession with a wild kick, the striker lifted his shot over the 'keeper and Craig Forsyth failed to prevent the ball from crossing the line.
And the former non-league star doubled up from the penalty spot ten minutes later, sending Grant the wrong way after Martin's shot hit the arm of Keogh, who was furious with the decision.
Millwall almost made it 3-0 in the 39th minute when Gregory's cross was headed clear at the near post, with Woolford waiting to nod the ball in.
Ince, though, hauled Derby back into the game with a deft 20-yard free-kick that curled around the defensive wall and bounced into the bottom corner.
The Lions needed to start the second half on the front foot and, five minutes after the restart, they restored their two-goal advantage.
Referee David Coote pointed to the spot after Dunne was pushed to the ground as he went to attack Martin's corner, and Gregory powered his shot beyond Grant's full-stretch dive.
The visitors looked to rally again and Johnny Russell and Richard Dawkins both went close during a hectic five-minute spell.
The momentum of the game shifted again when, in the 70th minute, Ince went down as soon as he felt contact from O'Brien, and sub Martin dispatched the penalty with aplomb.
Millwall quickly set about trying to score a fourth goal, with O'Brien's powerful drive and Woolford's glancing header both being tipped over by Grant.
But Harris' men couldn't hold on, and Hendrick finished off a superb team move to make it 3-3 with five minutes to go.
Both sides had chances to win it during added time but Fuller was thwarted by Keogh's diving lunge and Ince curled inches wide from 20 yards.