Millwall came from a goal behind to beat Chesterfield 2-1 and climb into the Sky Bet League One Play-Off places.
Sam Morsy had given the hosts a 25th-minute lead, but The Lions quickly equalised through Aiden O'Brien before Lee Gregory struck what turned out to be the winning goal early on in the second-half.
It was the home side who created the first opening of the game when Drew Talbot's cross from the left was headed clear by Byron Webster before Jed Wallace stung the palms of Tommy Lee with a shot from distance at the other end.
The opening exchanges were end-to-end as both sides looked to establish a foot hold in the match and that could have been Chesterfield's after 11 minutes when Sam Hird headed Jay O'Shea's corner narrowly over the bar from close range. Moments later, Morsy came even closer with a dipping drive from 25 yards.
Those two opportunities sparked a spell of pressure from Danny Wilson's side, who again came close when Lee Novak and Sylvan Ebanks-Blake exchanged passes before the latter fired wide from just outside the six-yard box.
Novak was then set through on goal only to hit the side netting with just Archer to beat but that was only a temporary reprieve for The Lions as Chesterfield took the lead. It came through central midfielder Morsy, who controlled well before finding the bottom left-hand corner with a powerful shot from distance after 25 minutes.
The home side's lead lasted barely 120 seconds, however, as O'Brien fired the visitors level. The Irishman reacted quickest to a ball from the left, getting enough power with his head to find the corner of the net despite Lee's best efforts.
Determined to quickly regain their advantage, Chesterfield went straight back on the attack but Archer was equal to O'Shea's volley before Novak's shot from distance was deflected wide.
It remained level at half-time but the game was turned on its head inside two minutes of the second period. A ball forward was nodded down by Steve Morison to Wallace, who turned it to Gregory and the striker then calmly slotted past Lee, sending the 854 travelling Millwall fans into raptures behind the goal.
The Lions could and maybe should have added a third a minute later when Wallace again found Gregory, but this time the striker headed over at the near post.
As Chesterfield opened up in search of an equaliser, Millwall enjoyed more and more space in the final third and the home side were almost made to pay when, on the hour mark, Morison teed up Gregory, who fired into the side netting.
There was a warning for the visitors after 71 minutes, though, as Morsy again let fly from distance, but this time Archer saved well before quickly gathering the loose ball.
Wilson replaced Ebanks-Blake with Byron Harrison in an attempt to find the equaliser and the striker did indeed come close to doing so, only to see a header kept out by Archer with a superb reaction stop after 81 minutes.
At the other end, Ed Upson, on in place of goalscorer O'Brien, found Morison, whose deflected cross was tipped onto the bar by Lee with the loose ball then cleared.
Chesterfield huffed and puffed in the closing stages but The Lions held firm and secured a victory which lifted them into the League's top six.
: Lee, O'Neil, Hird, Raglan, Talbot (Dieseruvwe, 87'), O'Shea, Morsy, Donohue, Ariyibi (Gardner, 65'), Novak, Ebanks-Blake (Harrison, 72')
Subs not used: Humphreys, Chapman, Onovwigun, Simons
Goalscorers: Morsy 25'
Millwall: Archer, Cummings, Webster, Beevers, Ferguson (J.Martin, 76'), Wallace, Abdou, Williams, O'Brien (Upson, 78'), Gregory (Onyedinma, 89'), Morison
Subs not used: Forde, Nelson, Thompson, Marquis
Goalscorers: O'Brien 27', Gregory 47'
Bookings: Morison 45'
Referee: Christopher Sarginson
Attendance: 7,078 (854)