MILLWALL boss Neil Harris has warned his players they will be facing a side "on the up" at Proact Stadium on Saturday afternoon.
Chesterfield handed Millwall their fourth successive home defeat when they won 2-0 at The Den at the end of August, but Dean Saunders' side then failed to win in September.
Their form in October improved and they sat 10th in the table after a 1-0 win at Fleetwood.
However, results nosedived again in November and Saunders was sacked at the end of the month after a 4-0 defeat at home to Swindon, their fourth league defeat in a row.
There has been a 15-point swing between the two sides since their meeting earlier in the season, but since the appointment of former Sheffield Wednesday manager Danny Wilson on Christmas Eve, the Spireites have collected seven points from five league games and scored 12 goals.
They defeated Shrewsbury 7-1 in their last game at home, and Harris is ready for a difficult contest.
"There's been a big change, after that game they certainly struggled and it cost Dean his job," Harris said.
"Danny Wilson has gone in and they've had a revival and are on the up. We've seen them play and they're a different opposition now to a few weeks ago.
"It's going to be a tough one, it's an aggressive team with some good players.
"They've got players with top-level experience: (Sylvan) Ebanks-Blake, (Lee) Novak. They've got players coming through like (Jay ) O'Shea (who scored twice in that 2-0 win) and Sam Morsy, who are good footballers.
"It's a challenge we're looking forward to because we feel we're just building some momentum. It's a key game for us and after two wins we're certainly looking to be really difficult opposition for Chesterfield, play with the flair we've shown recently but also try and be consistent with defending."
A win could lift Millwall into the top six if other results go their way, and Harris admits it's important to increase the pressure on the front runners.
"We've still got a long way to go as a team," he admitted. "With 20 games to go, just under half a season, it's getting quite exciting. We need to up our levels, definitely, to see if we can force our way into the top six.
"We still don't look too far ahead of ourselves, we still talk about winning the next game, and concentrate on Chesterfield next.
"We want the teams ahead of us to start looking over their shoulders at Millwall, Sheffield United and teams like that. The only way you do that is by winning games. We have to focus on ourselves and use the experience that some of our players have. But we also rely on the youthfulness and freshness that some of the boys have at this stage of the season that can give us added impetus."
The Lions are primed for a promotion push and have a more favourable run of fixtures from now until May 8 with all of the awkward Tuesday night league games away from home out of the way.
Asked if the club are currently in the position he expected before the season began, Harris reflected: "I never got that far in front. There was so much change needed in the summer – on and off the pitch – that we just needed to try and stabilise and re-focus and see where it took us.
"We changed a lot in a short space of time, certainly off the pitch as well: training methods, routines. Mentally and physically, an overhaul was needed and we're quietly pleased with how we've done.
"But I think it's clear to see we could be so much better.
"We're certainly not going to pat ourselves on the back, we've got so much more improvement in us. That's got to be the drive and the determination is to keep improving because if we were more consistent we could be sitting here with a much better points tally.
"The time for assessing is not now, it'll be towards the end of the season."